diff --git a/ajax/editActivity.php b/ajax/editActivity.php
index 57b01f7a5..4fc434654 100755
--- a/ajax/editActivity.php
+++ b/ajax/editActivity.php
@@ -65,9 +65,7 @@
 $activityHour = trim(urldecode($_POST['hour']));
 $activityMinute = trim(urldecode($_POST['minute']));
 
-$dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-    ? DATE_FORMAT_DDMMYY
-    : DATE_FORMAT_MMDDYY;
+$dateFormatFlag = $_SESSION['CATS']->getDateFormat();
 
 if (!DateUtility::validate('-', $activityDate, $dateFormatFlag))
 {
diff --git a/installwizard.php b/installwizard.php
index 7466d0ab5..05a282556 100755
--- a/installwizard.php
+++ b/installwizard.php
@@ -375,6 +375,7 @@
                                 <select id="dateFormat" name="dateFormat" class="form-select">
                                     <option value="mdy" selected="selected">MM-DD-YYYY (US)</option>
                                     <option value="dmy">DD-MM-YYYY (UK)</option>
+                                    <option value="ymd">YYYY-MM-DD (ISO)</option>
                                 </select></section>
                             <section class="mb-3"><label for="timeFormat" class="form-label">Please choose your preferred time format.</label>
                                 <select id="timeFormat" name="timeFormat" class="form-select">
diff --git a/js/calendarDateInput.js b/js/calendarDateInput.js
index 7510a4489..3ae9057d2 100755
--- a/js/calendarDateInput.js
+++ b/js/calendarDateInput.js
@@ -937,12 +937,7 @@ function SetDateInputDate(DateName, DateFormat, NewDate)
     /* Select the day. */
     dayList.selectedIndex = (day - 1);
 
-    /* Set the year. */
-    var yearString = (year % 100) + '';
-    if (yearString.length == 1)
-    {
-        yearString = '0' + yearString;
-    }
-
-    yearField.value = yearString;
+    /* Set the year (2 or 4 digits, matching the date format). */
+    yearField.value = object.picked.yearPad;
+    yearField.defaultValue = yearField.value;
 }
diff --git a/lib/Candidates.php b/lib/Candidates.php
index 26b6e9a7d..9f8d3d401 100755
--- a/lib/Candidates.php
+++ b/lib/Candidates.php
@@ -1529,18 +1529,20 @@ public function mergeDuplicates($params, $rs)
             );
         }
 
-        if (!empty($rs['dateAvailable']))
-        {
-            $dateParts = explode('-', $rs['dateAvailable']);
+        /* dateAvailable was formatted by the query in the session's date format. */
+        $dateFormat = isset($_SESSION['CATS'])
+            ? $_SESSION['CATS']->getDateFormat()
+            : DATE_FORMAT_MMDDYY;
 
-            if (count($dateParts) == 3)
-            {
-                $dateAvailable = '20' . $dateParts[2] . '-' .
-                $dateParts[0] . '-' . $dateParts[1] . ' 00:00:00';
+        if (!empty($rs['dateAvailable']) &&
+            DateUtility::validate('-', $rs['dateAvailable'], $dateFormat))
+        {
+            $dateAvailable = DateUtility::convert(
+                '-', $rs['dateAvailable'], $dateFormat, DATE_FORMAT_YYYYMMDD
+            ) . ' 00:00:00';
 
-                $updates[] = 'date_available = ' .
-                $this->_db->makeQueryString($dateAvailable);
-            }
+            $updates[] = 'date_available = ' .
+            $this->_db->makeQueryString($dateAvailable);
         }
 
         $sql = sprintf(
diff --git a/lib/Dashboard.php b/lib/Dashboard.php
index 301dd1384..149b3a212 100755
--- a/lib/Dashboard.php
+++ b/lib/Dashboard.php
@@ -200,14 +200,12 @@ public function getPipelineData($view)
             case DASHBOARD_GRAPH_WEEKLY:
 		    default:
               // TODO:   Localization d/m, week starts on monday
-                if ($_SESSION['CATS']->isDateDMY())
+                $pattern = match ($_SESSION['CATS']->getDateFormat())
                 {
-                    $pattern = "d/m";
-                }
-                else
-                {
-                    $pattern = "m/d";
-                }            
+                    DATE_FORMAT_DDMMYY   => "d/m",
+                    DATE_FORMAT_YYYYMMDD => "m-d",
+                    default              => "m/d",
+                };
             
                 /* * 6 at the end gives us the last day in the week (first day in week plus 6 days) */
                 $data[$rsCurrentTime['currentWeekNumber']] = array('label' => date($pattern, $rsCurrentTime['currentWeekNumber']) . ' - ' . date($pattern, $rsCurrentTime['currentWeekNumber'] + $oneUnixDay * 6));
diff --git a/lib/DatabaseConnection.php b/lib/DatabaseConnection.php
index 05acee0da..70656936c 100755
--- a/lib/DatabaseConnection.php
+++ b/lib/DatabaseConnection.php
@@ -20,7 +20,7 @@ class DatabaseConnection
     private $_connection = null;
     private $_queryResult = null;
     private $_timeZone;
-    private $_dateDMY;
+    private $_dateFormat;
     private $_inTransaction;
 
 
@@ -42,12 +42,12 @@ public static function getInstance()
         if (isset($_SESSION['CATS']) && $_SESSION['CATS']->isLoggedIn())
         {
             self::$_instance->_timeZone = $_SESSION['CATS']->getTimeZoneOffset();
-            self::$_instance->_dateDMY = $_SESSION['CATS']->isDateDMY();
+            self::$_instance->_dateFormat = $_SESSION['CATS']->getDateFormat();
         }
         else
         {
             self::$_instance->_timeZone = OFFSET_GMT * -1;
-            self::$_instance->_dateDMY = false;
+            self::$_instance->_dateFormat = DATE_FORMAT_MMDDYY;
         }
 
         return self::$_instance;
@@ -679,14 +679,21 @@ private function _localizationFilter($query)
 
         $query = $newQuery;
 
-        /* Replace m-d-y dates with d-m-y dates if we're in dmy mode. */
-        if ($this->_dateDMY)
+        /* Replace m-d-y dates with d-m-y or Y-m-d dates if we're in dmy or
+         * ymd mode.
+         */
+        if ($this->_dateFormat == DATE_FORMAT_DDMMYY)
         {
             $query = str_replace('%m-%d-%y', '%d-%m-%y', $query);
             $query = str_replace('%m-%d-%Y', '%d-%m-%Y', $query);
             $query = str_replace('%m/%d/%Y', '%d/%m/%Y', $query);
             $query = str_replace('%m/%d/%y', '%d/%m/%y', $query);
         }
+        else if ($this->_dateFormat == DATE_FORMAT_YYYYMMDD)
+        {
+            $query = str_replace(array('%m-%d-%y', '%m-%d-%Y'), '%Y-%m-%d', $query);
+            $query = str_replace(array('%m/%d/%y', '%m/%d/%Y'), '%Y/%m/%d', $query);
+        }
 
         return $query;
     }
diff --git a/lib/DateUtility.php b/lib/DateUtility.php
index 1f4d0734d..a14ebf6a7 100755
--- a/lib/DateUtility.php
+++ b/lib/DateUtility.php
@@ -396,6 +396,125 @@ public static function getDateTimeFormat($dateFormat, $isTimeFormat24 = null)
         return $dateFormat . ' ' . self::getTimeFormat($isTimeFormat24);
     }
 
+    /**
+     * Returns the date format flag for a date format flag or a legacy
+     * boolean "use D-M-Y" value (true = D-M-Y, false = M-D-Y).
+     *
+     * @param integer|boolean $dateFormat  date format flag or legacy boolean
+     * @return integer date format flag
+     */
+    public static function normalizeDateFormat($dateFormat)
+    {
+        if (is_bool($dateFormat))
+        {
+            return $dateFormat ? DATE_FORMAT_DDMMYY : DATE_FORMAT_MMDDYY;
+        }
+
+        return $dateFormat;
+    }
+
+    /**
+     * Returns the date format flag for a site.date_format_ddmmyy value
+     * (0 = M-D-Y, 1 = D-M-Y, 2 = Y-M-D).
+     *
+     * @param integer $siteValue  site.date_format_ddmmyy value
+     * @return integer DATE_FORMAT_MMDDYY, DATE_FORMAT_DDMMYY or DATE_FORMAT_YYYYMMDD
+     */
+    public static function getDateFormatFromSiteValue($siteValue)
+    {
+        return match ((int) $siteValue)
+        {
+            1       => DATE_FORMAT_DDMMYY,
+            2       => DATE_FORMAT_YYYYMMDD,
+            default => DATE_FORMAT_MMDDYY,
+        };
+    }
+
+    /**
+     * Returns the site.date_format_ddmmyy value for a date format flag.
+     *
+     * @param integer $dateFormat  date format flag
+     * @return integer site.date_format_ddmmyy value
+     */
+    public static function getSiteValueFromDateFormat($dateFormat)
+    {
+        return match ($dateFormat)
+        {
+            DATE_FORMAT_DDMMYY   => 1,
+            DATE_FORMAT_YYYYMMDD => 2,
+            default              => 0,
+        };
+    }
+
+    /**
+     * Returns the date format flag for a localization form value
+     * ('mdy', 'dmy' or 'ymd').
+     *
+     * @param string $formValue  localization form value
+     * @return integer date format flag
+     */
+    public static function getDateFormatFromFormValue($formValue)
+    {
+        return match ($formValue)
+        {
+            'dmy'   => DATE_FORMAT_DDMMYY,
+            'ymd'   => DATE_FORMAT_YYYYMMDD,
+            default => DATE_FORMAT_MMDDYY,
+        };
+    }
+
+    /**
+     * Returns the calendarDateInput.js format string for a date format flag.
+     *
+     * @param integer|null $dateFormat  date format flag, null=read from session
+     * @return string 'MM-DD-YY', 'DD-MM-YY' or 'YYYY-MM-DD'
+     */
+    public static function getJsDateFormat($dateFormat = null)
+    {
+        return match ($dateFormat ?? self::_getSessionDateFormat())
+        {
+            DATE_FORMAT_DDMMYY   => 'DD-MM-YY',
+            DATE_FORMAT_YYYYMMDD => 'YYYY-MM-DD',
+            default              => 'MM-DD-YY',
+        };
+    }
+
+    /**
+     * Returns the PHP date() format string for a date format flag.
+     *
+     * @param integer|null $dateFormat  date format flag, null=read from session
+     * @return string 'm-d-y', 'd-m-y' or 'Y-m-d'
+     */
+    public static function getPhpDateFormat($dateFormat = null)
+    {
+        return match ($dateFormat ?? self::_getSessionDateFormat())
+        {
+            DATE_FORMAT_DDMMYY   => 'd-m-y',
+            DATE_FORMAT_YYYYMMDD => 'Y-m-d',
+            default              => 'm-d-y',
+        };
+    }
+
+    /**
+     * Converts a date stored as MM-DD-YY (e.g. extra field values) to the
+     * specified date format. Values that are not valid MM-DD-YY dates are
+     * returned unchanged.
+     *
+     * @param string  $date        MM-DD-YY date
+     * @param integer $dateFormat  date format flag
+     * @return string date in $dateFormat format
+     */
+    public static function formatStoredDate($date, $dateFormat)
+    {
+        if ($dateFormat == DATE_FORMAT_MMDDYY ||
+            !self::validate('-', (string) $date, DATE_FORMAT_MMDDYY))
+        {
+            return $date;
+        }
+
+        return self::convert('-', $date, DATE_FORMAT_MMDDYY, $dateFormat);
+    }
+
     /**
      * Returns the MySQL DATE_FORMAT time-only format string for use inside
      * a PHP sprintf() call (% signs are already doubled).
@@ -838,6 +957,13 @@ public function getPeriodDateRange($period, $dateFormat = false)
         );
     }
 
+    private static function _getSessionDateFormat()
+    {
+        return isset($_SESSION['CATS'])
+            ? $_SESSION['CATS']->getDateFormat()
+            : DATE_FORMAT_MMDDYY;
+    }
+
     private static function _removeLeadingZeros($array)
     {
         foreach ($array as $key => $value)
diff --git a/lib/EmailTemplates.php b/lib/EmailTemplates.php
index 75f2c4592..d5799b0c8 100755
--- a/lib/EmailTemplates.php
+++ b/lib/EmailTemplates.php
@@ -210,14 +210,7 @@ public function replaceVariables($text)
         $siteName = $_SESSION['CATS']->getSiteName();
         $fullName = $_SESSION['CATS']->getFullName();
 
-        if ($_SESSION['CATS']->isDateDMY())
-        {
-            $dateFormat = 'd-m-y';
-        }
-        else
-        {
-            $dateFormat = 'm-d-y';
-        }
+        $dateFormat = DateUtility::getPhpDateFormat();
 
         if (isset($_SESSION['CATS']))
         {
diff --git a/lib/ExtraFields.php b/lib/ExtraFields.php
index e4dd2331c..3dd296f76 100755
--- a/lib/ExtraFields.php
+++ b/lib/ExtraFields.php
@@ -486,25 +486,9 @@ public function getValuesForShow($dataItemID)
                 break;
                 
                 case EXTRA_FIELD_DATE:
-                    $dmy = $this->_isDateDMY();
-                    
-                    if ($dmy)
-                    {
-                        $dateParts = explode('-', $extraFields[$index]['value']);
-                        if (count($dateParts) > 2)
-                        {
-                            $t = $dateParts[0];
-                            $dateParts[0] = $dateParts[1];
-                            $dateParts[1] = $t;
-                        }
-                        $date = implode('-', $dateParts);
-                        
-                        $extraFields[$index]['display'] = htmlspecialchars($date);
-                    }
-                    else
-                    {
-                        $extraFields[$index]['display'] = htmlspecialchars($extraFields[$index]['value']);
-                    }
+                    $extraFields[$index]['display'] = htmlspecialchars(DateUtility::formatStoredDate(
+                        $extraFields[$index]['value'], $this->_getDateFormat()
+                    ));
                 break;
                 
                 case EXTRA_FIELD_TEXT:
@@ -589,7 +573,7 @@ public function getValuesForAdd()
                 break;
                 
                 case EXTRA_FIELD_DATE:
-                    $dateFormat = $this->_isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY';
+                    $dateFormat = DateUtility::getJsDateFormat($this->_getDateFormat());
                     $extraFields[$index]['addHTML'] = '<script type="text/javascript">DateInput(\'extraField'.$index.'\', false, \''.$dateFormat.'\', \'\');</script>';
                 break;
                 
@@ -661,33 +645,17 @@ public function getDataGridDefinition($uniqueIndex, $data, $db)
                                        'ON '.$column.' = extra_field' . $uniqueIndex . '.data_item_id '.
                                        'AND extra_field' . $uniqueIndex . '.field_name = ' . $db->makeQueryString($data['fieldName']) . ' '.
                                        'AND extra_field' . $uniqueIndex . '.data_item_type = ' . $this->_dataItemType,
-                          'pagerRender'     => 'if (isset($_SESSION[\'CATS\']) && $_SESSION[\'CATS\']->isLoggedIn() && $_SESSION[\'CATS\']->isDateDMY())
+                          'pagerRender'     => 'if (isset($_SESSION[\'CATS\']) && $_SESSION[\'CATS\']->isLoggedIn())
                                         {
-                                              $dateParts = explode(\'-\',  $rsData[\'extra_field_value' . $uniqueIndex . '\']);
-                                              if (count($dateParts) > 2)
-                                              {
-                                                    $t = $dateParts[0];
-                                                    $dateParts[0] = $dateParts[1];
-                                                    $dateParts[1] = $t;
-                                              }
-                                              $date = implode(\'-\', $dateParts);
-                                              return $date;
+                                              return DateUtility::formatStoredDate($rsData[\'extra_field_value' . $uniqueIndex . '\'], $_SESSION[\'CATS\']->getDateFormat());
                                         }
                                         else
                                         {
                                              return $rsData[\'extra_field_value' . $uniqueIndex . '\'];
                                         }',
-                          'exportRender'     => 'if (isset($_SESSION[\'CATS\']) && $_SESSION[\'CATS\']->isLoggedIn() && $_SESSION[\'CATS\']->isDateDMY())
+                          'exportRender'     => 'if (isset($_SESSION[\'CATS\']) && $_SESSION[\'CATS\']->isLoggedIn())
                                         {
-                                              $dateParts = explode(\'-\',  $rsData[\'extra_field_value' . $uniqueIndex . '\']);
-                                              if (count($dateParts) > 2)
-                                              {
-                                                    $t = $dateParts[0];
-                                                    $dateParts[0] = $dateParts[1];
-                                                    $dateParts[1] = $t;
-                                              }
-                                              $date = implode(\'-\', $dateParts);
-                                              return $date;
+                                              return DateUtility::formatStoredDate($rsData[\'extra_field_value' . $uniqueIndex . '\'], $_SESSION[\'CATS\']->getDateFormat());
                                         }
                                         else
                                         {
@@ -780,14 +748,9 @@ public function getValuesForEdit($dataItemID)
                 break;
                 
                 case EXTRA_FIELD_DATE:
-                    $userDateFormat = $this->_isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY';
-                    $userDateValue = $data['value'];
-                    if (!empty($userDateValue) && $userDateFormat == 'DD-MM-YY')
-                    {
-                        $userDateValue = DateUtility::convert(
-                            '-', $userDateValue, DATE_FORMAT_MMDDYY, DATE_FORMAT_DDMMYY
-                        );
-                    }
+                    $dateFormatFlag = $this->_getDateFormat();
+                    $userDateFormat = DateUtility::getJsDateFormat($dateFormatFlag);
+                    $userDateValue = DateUtility::formatStoredDate($data['value'], $dateFormatFlag);
                     $extraFields[$index]['editHTML'] = '<script type="text/javascript">DateInput(\'extraField'.$index.'\', false, \''.$userDateFormat.'\', \''.htmlspecialchars($userDateValue, ENT_QUOTES).'\');</script>';
                 break;
                                     
@@ -814,9 +777,7 @@ public function setValuesOnEdit($dataItemID)
     {
         $extraFields = $this->_getValuesWithSettings($dataItemID);
 
-        $dateFormatFlag = $this->_isDateDMY()
-            ? DATE_FORMAT_DDMMYY
-            : DATE_FORMAT_MMDDYY;
+        $dateFormatFlag = $this->_getDateFormat();
 
         for ($i = 0; $i < count($extraFields); $i++)
         {
@@ -847,9 +808,19 @@ public function setValuesOnEdit($dataItemID)
 
                 if (!empty($newValue))
                 {
-                    $newValue = DateUtility::convert(
+                    $storedValue = DateUtility::convert(
                         '-', $newValue, $dateFormatFlag, DATE_FORMAT_MMDDYY
                     );
+
+                    /* Storage is MM-DD-YY; reject dates whose century would
+                     * be lost (e.g. 1950-06-12 would read back as 2050).
+                     */
+                    if (DateUtility::formatStoredDate($storedValue, $dateFormatFlag) !== $newValue)
+                    {
+                        continue;
+                    }
+
+                    $newValue = $storedValue;
                 }
             }
 
@@ -870,18 +841,18 @@ public function setValuesOnEdit($dataItemID)
      *
      * @return array extra fields
      */
-    private function _isDateDMY()
+    private function _getDateFormat()
     {
         if (isset($_SESSION['CATS']) && $_SESSION['CATS']->isLoggedIn())
         {
-            return $_SESSION['CATS']->isDateDMY();
+            return $_SESSION['CATS']->getDateFormat();
         }
 
         /* Careers portal / unauthenticated: fall back to site preference. */
         $site = new Site();
         $siteRS = $site->getFirstSite();
 
-        return ($siteRS['dateFormatDDMMYY'] == 1);
+        return DateUtility::getDateFormatFromSiteValue($siteRS['dateFormatDDMMYY']);
     }
 
     public static function getValuesTypes()
diff --git a/lib/Session.php b/lib/Session.php
index d082f9f9a..aae7f209a 100755
--- a/lib/Session.php
+++ b/lib/Session.php
@@ -10,6 +10,7 @@
  */
 
 include(LEGACY_ROOT . '/lib/ACL.php');
+include_once(LEGACY_ROOT . '/lib/DateUtility.php');
 
 /**
  *  CATS Session Object
@@ -49,7 +50,10 @@ class CATSSession
     private $_timeZoneOffset = 0;
     private $_timeZone = 0;
     private $_defaultPhoneCountryCode = '+1';
-    private $_dateDMY = false;
+    private $_dateFormat = DATE_FORMAT_MMDDYY;
+    /* Deprecated: only set in sessions serialized before _dateFormat was
+     * added; migrated to _dateFormat by __wakeup(). */
+    private $_dateDMY = null;
     private $_timeFormat24 = false;
     private $_pipelineEntriesPerPage = 15;
     private $_storedData = array();
@@ -334,7 +338,34 @@ public function getUserAgent()
      */
     public function isDateDMY()
     {
-        return $this->_dateDMY;
+        return ($this->_dateFormat == DATE_FORMAT_DDMMYY);
+    }
+
+    /**
+     * Returns the date format flag for the current site (DATE_FORMAT_MMDDYY,
+     * DATE_FORMAT_DDMMYY or DATE_FORMAT_YYYYMMDD). The database is not
+     * accessed.
+     *
+     * @return integer date format flag
+     */
+    public function getDateFormat()
+    {
+        return $this->_dateFormat;
+    }
+
+    /**
+     * Migrates the date format of sessions serialized before _dateFormat was
+     * added, so users logged in during an upgrade keep their date format.
+     *
+     * @return void
+     */
+    public function __wakeup()
+    {
+        if ($this->_dateDMY !== null)
+        {
+            $this->_dateFormat = DateUtility::normalizeDateFormat((bool) $this->_dateDMY);
+            $this->_dateDMY = null;
+        }
     }
 
     /**
@@ -557,16 +588,16 @@ public function setDropdown($name, $value)
      * is not modified.
      *
      * @param integer Time zone offset from GMT.
-     * @param boolean Display dates in D-M-Y format?
+     * @param integer|boolean Date format flag, or legacy boolean D-M-Y value.
      * @return void
      */
-    public function setTimeDateLocalization($timeZone, $isDMY, $isTimeFormat24 = false)
+    public function setTimeDateLocalization($timeZone, $dateFormat, $isTimeFormat24 = false)
     {
         $timeZone = (integer) $timeZone;
 
         $this->_timeZone       = $timeZone;
         $this->_timeZoneOffset = $timeZone - OFFSET_GMT;
-        $this->_dateDMY        = $isDMY;
+        $this->_dateFormat     = DateUtility::normalizeDateFormat($dateFormat);
         $this->_timeFormat24   = (bool) $isTimeFormat24;
     }
 
@@ -766,7 +797,7 @@ public function processLogin($username, $password, $addToHistory = true)
                 $this->_timeZoneOffset         = $rs['timeZone'] - OFFSET_GMT;
                 $this->_timeZone               = $rs['timeZone'];
                 $this->_defaultPhoneCountryCode = $rs['defaultPhoneCountryCode'];
-                $this->_dateDMY                = ($rs['dateFormatDMY'] == 0 ? false : true);
+                $this->_dateFormat             = DateUtility::getDateFormatFromSiteValue($rs['dateFormatDMY']);
                 $this->_timeFormat24           = ($rs['timeFormat24'] == 0 ? false : true);
                 $this->_canSeeEEOInfo          = ($rs['canSeeEEOInfo'] == 0 ? false : true);
                 $this->_pipelineEntriesPerPage = $rs['pipelineEntriesPerPage'];
diff --git a/lib/Site.php b/lib/Site.php
index 810742ba5..e4c071f70 100755
--- a/lib/Site.php
+++ b/lib/Site.php
@@ -9,6 +9,8 @@
  * See LICENSE.md.
  */
 
+include_once(LEGACY_ROOT . '/lib/DateUtility.php');
+
 /**
  *	Site Library
  *	@package    CATS
@@ -48,12 +50,14 @@ public function setName($name)
      * Sets the site name for the current site.
      *
      * @param integer time zone offset
-     * @param boolean use D-M-Y format dates
+     * @param integer|boolean date format flag, or legacy boolean D-M-Y value
      * @param boolean use 24-hour time format
      * @return boolean True if successful; false otherwise.
      */
-    public function setLocalization($timeZone, $isDMY, $isTimeFormat24 = false)
+    public function setLocalization($timeZone, $dateFormat, $isTimeFormat24 = false)
     {
+        $dateFormat = DateUtility::normalizeDateFormat($dateFormat);
+
         $sql = sprintf(
             "UPDATE
                 site
@@ -62,7 +66,7 @@ public function setLocalization($timeZone, $isDMY, $isTimeFormat24 = false)
                 date_format_ddmmyy = %s,
                 time_format_24 = %s",
             $this->_db->makeQueryInteger($timeZone),
-            ($isDMY ? 1 : 0),
+            DateUtility::getSiteValueFromDateFormat($dateFormat),
             ($isTimeFormat24 ? 1 : 0)
         );
 
diff --git a/modules/activity/AddActivityScheduleEventModal.tpl b/modules/activity/AddActivityScheduleEventModal.tpl
index aa71a66c1..a35028e08 100644
--- a/modules/activity/AddActivityScheduleEventModal.tpl
+++ b/modules/activity/AddActivityScheduleEventModal.tpl
@@ -8,7 +8,7 @@
 <?php if (!$this->isFinishedMode): ?>
 
 <script type="text/javascript">
-    window.CATSUserDateFormat = '<?php echo($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>';
+    window.CATSUserDateFormat = '<?php echo(DateUtility::getJsDateFormat()); ?>';
 </script>
 
     <form name="logActivityForm" id="logActivityForm" action="<?php echo(CATSUtility::getIndexName()); ?>?m=<?php echo($this->activityParentModule); ?>&amp;a=<?php echo($this->activitySubmitAction); ?><?php if ($this->onlyScheduleEvent): ?>&amp;onlyScheduleEvent=true<?php endif; ?>" method="post" onsubmit="return checkActivityForm(document.logActivityForm);" autocomplete="off">
diff --git a/modules/calendar/Calendar.tpl b/modules/calendar/Calendar.tpl
index 04650a5c6..832851732 100755
--- a/modules/calendar/Calendar.tpl
+++ b/modules/calendar/Calendar.tpl
@@ -2,7 +2,7 @@
 <?php TemplateUtility::printHeaderBlock(); ?>
 <?php TemplateUtility::printTabs($this->active); ?>
     <script>
-        window.CATSUserDateFormat = '<?php echo($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>';
+        window.CATSUserDateFormat = '<?php echo(DateUtility::getJsDateFormat()); ?>';
         window.CATSTimeFormat24 = <?php echo($_SESSION['CATS']->isTimeFormat24() ? 'true' : 'false'); ?>;
     </script>
 <?php TemplateUtility::printQuickSearch(); ?>
@@ -48,7 +48,7 @@
                                 </div>
                                 <div class="mb-2">
                                     <label class="form-label mb-1" id="dateLabel" for="dateAdd_Month_ID">Date:</label>
-                                    <script>DateInput('dateAdd', true, (typeof window.CATSUserDateFormat !== 'undefined' ? window.CATSUserDateFormat : 'MM-DD-YY'), '<?php echo($_SESSION['CATS']->isDateDMY() ? DateUtility::getAdjustedDate('d-m-y') : $this->currentDateMDY); ?>', -1);</script>
+                                    <script>DateInput('dateAdd', true, (typeof window.CATSUserDateFormat !== 'undefined' ? window.CATSUserDateFormat : 'MM-DD-YY'), '<?php echo(DateUtility::getAdjustedDate(DateUtility::getPhpDateFormat())); ?>', -1);</script>
                                 </div>
                                 <div class="mb-2">
                                     <label class="form-label mb-1" id="timeLabel" for="hour">Time:</label>
@@ -156,7 +156,7 @@
                                 </div>
                                 <div class="mb-2">
                                     <label class="form-label mb-1" id="dateLabelEdit" for="dateEdit_Month_ID">Date:</label>
-                                    <script>DateInput('dateEdit', true, (typeof window.CATSUserDateFormat !== 'undefined' ? window.CATSUserDateFormat : 'MM-DD-YY'), '<?php echo($_SESSION['CATS']->isDateDMY() ? DateUtility::getAdjustedDate('d-m-y') : $this->currentDateMDY); ?>', -1);</script>
+                                    <script>DateInput('dateEdit', true, (typeof window.CATSUserDateFormat !== 'undefined' ? window.CATSUserDateFormat : 'MM-DD-YY'), '<?php echo(DateUtility::getAdjustedDate(DateUtility::getPhpDateFormat())); ?>', -1);</script>
                                 </div>
                                 <div class="mb-2">
                                     <label class="form-label mb-1" id="timeLabelEdit" for="hourEdit">Time:</label>
diff --git a/modules/calendar/CalendarUI.js b/modules/calendar/CalendarUI.js
index 817e02f32..aa15eb11b 100755
--- a/modules/calendar/CalendarUI.js
+++ b/modules/calendar/CalendarUI.js
@@ -173,6 +173,8 @@ function calendarEditEvent(entry)
     var userDateFormat = (typeof window.CATSUserDateFormat !== "undefined" ? window.CATSUserDateFormat : "MM-DD-YY");
     var dateString = (userDateFormat == "DD-MM-YY"
         ? dayString + "-" + monthString + "-" + yearString
+        : userDateFormat == "YYYY-MM-DD"
+        ? GetGoodYear(yearString) + "-" + monthString + "-" + dayString
         : monthString + "-" + dayString + "-" + yearString);
     SetDateInputDate("dateEdit", userDateFormat, dateString);
 
@@ -372,6 +374,8 @@ function addEventByDay(year, month, day, hour)
     var userDateFormat = (typeof window.CATSUserDateFormat !== "undefined" ? window.CATSUserDateFormat : "MM-DD-YY");
     var dateString = (userDateFormat == "DD-MM-YY"
         ? dayString + "-" + monthString + "-" + yearString
+        : userDateFormat == "YYYY-MM-DD"
+        ? year + "-" + monthString + "-" + dayString
         : monthString + "-" + dayString + "-" + yearString);
     SetDateInputDate("dateAdd", userDateFormat, dateString);
 
diff --git a/modules/calendar/CalendarUI.php b/modules/calendar/CalendarUI.php
index 502d40797..cb65dd00a 100755
--- a/modules/calendar/CalendarUI.php
+++ b/modules/calendar/CalendarUI.php
@@ -86,7 +86,6 @@ private function showCalendar()
         $currentMonth    = DateUtility::getAdjustedDate('n');
         $currentYear     = DateUtility::getAdjustedDate('Y');
         $currentUnixTime = DateUtility::getAdjustedDate();
-        $currentDateMDY  = DateUtility::getAdjustedDate('m-d-y');
 
         $currentWeek  = DateUtility::getWeekNumber($currentUnixTime) - DateUtility::getWeekNumber(
             mktime(0, 0, 0, $currentMonth, 1, $currentYear)
@@ -260,7 +259,6 @@ private function showCalendar()
         $this->_template->assign('militaryTime', false);
 
         $this->_template->assign('active', $this);
-        $this->_template->assign('currentDateMDY', $currentDateMDY);
         $this->_template->assign('startingWeekday', $startingWeekday);
         $this->_template->assign('daysInMonth', $daysInMonth);
         $this->_template->assign('currentHour', $currentHour);
@@ -338,9 +336,7 @@ private function onAddEvent()
 
         /* Bail out if we received an invalid date. */
         $trimmedDate = $this->getTrimmedInput('dateAdd', $_POST);
-        $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-            ? DATE_FORMAT_DDMMYY
-            : DATE_FORMAT_MMDDYY;
+        $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
         if (empty($trimmedDate) ||
             !DateUtility::validate('-', $trimmedDate, $dateFormatFlag))
         {
@@ -550,9 +546,7 @@ private function onEditEvent()
 
         /* Bail out if we received an invalid date. */
         $trimmedDate = $this->getTrimmedInput('dateEdit', $_POST);
-        $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-            ? DATE_FORMAT_DDMMYY
-            : DATE_FORMAT_MMDDYY;
+        $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
         if (empty($trimmedDate) ||
             !DateUtility::validate('-', $trimmedDate, $dateFormatFlag))
         {
diff --git a/modules/candidates/Add.tpl b/modules/candidates/Add.tpl
index 259e15d7c..2aa8eca87 100755
--- a/modules/candidates/Add.tpl
+++ b/modules/candidates/Add.tpl
@@ -27,7 +27,7 @@
 <?php endif; ?>
 
 <script>
-    window.CATSUserDateFormat = '<?php echo($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>';
+    window.CATSUserDateFormat = '<?php echo(DateUtility::getJsDateFormat()); ?>';
 </script>
 
             <h2 class="h6 card-header bg-secondary-subtle py-1 px-2 fw-semibold mb-2">Basic Information</h2>
diff --git a/modules/candidates/CandidatesUI.php b/modules/candidates/CandidatesUI.php
index d4cc5b9db..db2c1bd75 100755
--- a/modules/candidates/CandidatesUI.php
+++ b/modules/candidates/CandidatesUI.php
@@ -1272,9 +1272,7 @@ private function onEdit()
          * ahead and convert the date to MySQL format.
          */
         $dateAvailable = $this->getTrimmedInput('dateAvailable', $_POST);
-        $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-            ? DATE_FORMAT_DDMMYY
-            : DATE_FORMAT_MMDDYY;
+        $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
         if (!empty($dateAvailable))
         {
             if (!DateUtility::validate('-', $dateAvailable, $dateFormatFlag))
@@ -2804,9 +2802,7 @@ private function _addCandidate($isModal, $directoryOverride = '')
          * ahead and convert the date to MySQL format.
          */
         $dateAvailable = $this->getTrimmedInput('dateAvailable', $_POST);
-        $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-            ? DATE_FORMAT_DDMMYY
-            : DATE_FORMAT_MMDDYY;
+        $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
         if (!empty($dateAvailable))
         {
             if (!DateUtility::validate('-', $dateAvailable, $dateFormatFlag))
@@ -3196,9 +3192,7 @@ private function _addActivity($isJobOrdersMode, $regardingID,
 
             $activityDateOccurred = false;
             $isTimeFormat24 = $_SESSION['CATS']->isTimeFormat24();
-            $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-                ? DATE_FORMAT_DDMMYY
-                : DATE_FORMAT_MMDDYY;
+            $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
             $activityDate = $this->getTrimmedInput('activityDate', $_POST);
             $activityHourSet = isset($_POST['activityHour']) && isset($_POST['activityMinute']) &&
                 ctype_digit((string) $_POST['activityHour']) &&
@@ -3249,9 +3243,7 @@ private function _addActivity($isJobOrdersMode, $regardingID,
         {
             /* Bail out if we received an invalid date. */
             $trimmedDate = $this->getTrimmedInput('dateAdd', $_POST);
-            $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-                ? DATE_FORMAT_DDMMYY
-                : DATE_FORMAT_MMDDYY;
+            $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
             if (empty($trimmedDate) ||
                 !DateUtility::validate('-', $trimmedDate, $dateFormatFlag))
             {
diff --git a/modules/candidates/Edit.tpl b/modules/candidates/Edit.tpl
index e5f4dac88..cacb88d33 100755
--- a/modules/candidates/Edit.tpl
+++ b/modules/candidates/Edit.tpl
@@ -2,7 +2,7 @@
 <?php TemplateUtility::printHeaderBlock(); ?>
 <?php TemplateUtility::printTabs($this->active); ?>
     <script>
-        window.CATSUserDateFormat = '<?php echo($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>';
+        window.CATSUserDateFormat = '<?php echo(DateUtility::getJsDateFormat()); ?>';
     </script>
     <?php TemplateUtility::printQuickSearch(); ?>
 <main id="main" class="container-fluid py-2 oc-candidate-edit-page">
diff --git a/modules/candidates/Show.tpl b/modules/candidates/Show.tpl
index 8fc635a07..b1e405041 100755
--- a/modules/candidates/Show.tpl
+++ b/modules/candidates/Show.tpl
@@ -25,7 +25,7 @@ use OpenCATS\UI\CandidateDuplicateQuickActionMenu;
 <?php endif; ?>
 
         <script>
-            window.CATSUserDateFormat  = <?php echo Template::escapeJs($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>;
+            window.CATSUserDateFormat  = <?php echo Template::escapeJs(DateUtility::getJsDateFormat()); ?>;
             window.CATSTimeFormat24    = <?php echo $_SESSION['CATS']->isTimeFormat24() ? 'true' : 'false'; ?>;
         </script>
 
diff --git a/modules/companies/Show.tpl b/modules/companies/Show.tpl
index 83ea9fae1..7f2b086ca 100755
--- a/modules/companies/Show.tpl
+++ b/modules/companies/Show.tpl
@@ -18,7 +18,7 @@ $extraFieldSplit = intdiv(count($this->extraFieldRS), 2);
 
 <script>
 window.CATSUserDateFormat = <?php echo Template::escapeJs(
-    $_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'
+    DateUtility::getJsDateFormat()
 ); ?>;
 window.CATSTimeFormat24 = <?php echo $_SESSION['CATS']->isTimeFormat24() ? 'true' : 'false'; ?>;
 </script>
diff --git a/modules/contacts/ContactsUI.php b/modules/contacts/ContactsUI.php
index 597fbe8aa..18b35057c 100755
--- a/modules/contacts/ContactsUI.php
+++ b/modules/contacts/ContactsUI.php
@@ -1381,9 +1381,7 @@ private function _addActivityScheduleEvent($regardingID, $directoryOverride = ''
 
             $activityDateOccurred = false;
             $isTimeFormat24 = $_SESSION['CATS']->isTimeFormat24();
-            $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-                ? DATE_FORMAT_DDMMYY
-                : DATE_FORMAT_MMDDYY;
+            $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
             $activityDate = $this->getTrimmedInput('activityDate', $_POST);
             $activityHourSet = isset($_POST['activityHour']) && isset($_POST['activityMinute']) &&
                 ctype_digit((string) $_POST['activityHour']) &&
@@ -1434,9 +1432,7 @@ private function _addActivityScheduleEvent($regardingID, $directoryOverride = ''
         {
             /* Bail out if we received an invalid date. */
             $trimmedDate = $this->getTrimmedInput('dateAdd', $_POST);
-            $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-                ? DATE_FORMAT_DDMMYY
-                : DATE_FORMAT_MMDDYY;
+            $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
             if (empty($trimmedDate) ||
                 !DateUtility::validate('-', $trimmedDate, $dateFormatFlag))
             {
diff --git a/modules/contacts/Show.tpl b/modules/contacts/Show.tpl
index 439085ce0..dd357119b 100755
--- a/modules/contacts/Show.tpl
+++ b/modules/contacts/Show.tpl
@@ -6,7 +6,7 @@ use OpenCATS\UI\QuickActionMenu;
 <?php TemplateUtility::printHeaderBlock(); ?>
 <?php TemplateUtility::printTabs($this->active); ?>
 <script>
-    window.CATSUserDateFormat  = <?php echo Template::escapeJs($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>;
+    window.CATSUserDateFormat  = <?php echo Template::escapeJs(DateUtility::getJsDateFormat()); ?>;
     window.CATSTimeFormat24    = <?php echo $_SESSION['CATS']->isTimeFormat24() ? 'true' : 'false'; ?>;
 </script>
 <?php TemplateUtility::printQuickSearch(); ?>
diff --git a/modules/install/ajax/ui.php b/modules/install/ajax/ui.php
index 495dc0cda..b83f79477 100755
--- a/modules/install/ajax/ui.php
+++ b/modules/install/ajax/ui.php
@@ -508,14 +508,13 @@
         /* Detect date format preferences. */
         $record = MySQLGetAssoc('SELECT date_format_ddmmyy FROM site LIMIT 1', true);
 
-        if (!isset($record['date_format_ddmmyy']) || $record['date_format_ddmmyy'] == 0)
-        {
-            echo 'document.getElementById(\'dateFormat\').value = \'mdy\';';
-        }
-        else
-        {
-            echo 'document.getElementById(\'dateFormat\').value = \'dmy\';';
-        }
+        $dateFormatValues = array(
+            DATE_FORMAT_MMDDYY   => 'mdy',
+            DATE_FORMAT_DDMMYY   => 'dmy',
+            DATE_FORMAT_YYYYMMDD => 'ymd'
+        );
+        $dateFormat = DateUtility::getDateFormatFromSiteValue($record['date_format_ddmmyy'] ?? 0);
+        echo 'document.getElementById(\'dateFormat\').value = \'' . $dateFormatValues[$dateFormat] . '\';';
 
         /* Detect default phone country calling code. */
         $defaultPhoneCountryCodeDigits = '';
@@ -946,16 +945,12 @@
          * (rows don't exist in schema till now.)
          */
 
-        $dateFormat = $_SESSION['dateFormatInstaller'];
+        $dateFormat = DateUtility::getDateFormatFromFormValue($_SESSION['dateFormatInstaller']);
 
-        if ($dateFormat == 'mdy')
-        {
-            MySQLQuery('UPDATE site SET date_format_ddmmyy = 0');
-        }
-        else
-        {
-            MySQLQuery('UPDATE site SET date_format_ddmmyy = 1');
-        }
+        MySQLQuery(sprintf(
+            'UPDATE site SET date_format_ddmmyy = %d',
+            DateUtility::getSiteValueFromDateFormat($dateFormat)
+        ));
 
         $timeFormat = isset($_SESSION['timeFormatInstaller']) ? $_SESSION['timeFormatInstaller'] : '12';
         MySQLQuery('UPDATE site SET time_format_24 = ' . ($timeFormat === '24' ? 1 : 0));
diff --git a/modules/joborders/Add.tpl b/modules/joborders/Add.tpl
index 47975454a..e4e4c823c 100755
--- a/modules/joborders/Add.tpl
+++ b/modules/joborders/Add.tpl
@@ -2,7 +2,7 @@
 <?php TemplateUtility::printHeaderBlock(); ?>
 <?php TemplateUtility::printTabs($this->active, $this->subActive); ?>
 <script>
-        window.CATSUserDateFormat = '<?php echo($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>';
+        window.CATSUserDateFormat = '<?php echo(DateUtility::getJsDateFormat()); ?>';
     </script>
 <?php TemplateUtility::printQuickSearch(); ?>
 <main id="main" class="container-fluid py-2 oc-joborder-add-page">
diff --git a/modules/joborders/Edit.tpl b/modules/joborders/Edit.tpl
index 773d1ba20..5cf85acc4 100755
--- a/modules/joborders/Edit.tpl
+++ b/modules/joborders/Edit.tpl
@@ -2,7 +2,7 @@
 <?php TemplateUtility::printHeaderBlock(); ?>
 <?php TemplateUtility::printTabs($this->active); ?>
 <script>
-        window.CATSUserDateFormat = '<?php echo($_SESSION['CATS']->isDateDMY() ? 'DD-MM-YY' : 'MM-DD-YY'); ?>';
+        window.CATSUserDateFormat = '<?php echo(DateUtility::getJsDateFormat()); ?>';
     </script>
 <?php TemplateUtility::printQuickSearch(); ?>
 <main id="main" class="container-fluid py-2 oc-joborder-edit-page">
diff --git a/modules/joborders/JobOrdersUI.php b/modules/joborders/JobOrdersUI.php
index fa23ea728..3e59a374a 100755
--- a/modules/joborders/JobOrdersUI.php
+++ b/modules/joborders/JobOrdersUI.php
@@ -763,9 +763,7 @@ private function onAdd()
          * convert the date to MySQL format.
          */
         $startDate = $this->getTrimmedInput('startDate', $_POST);
-        $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-            ? DATE_FORMAT_DDMMYY
-            : DATE_FORMAT_MMDDYY;
+        $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
         if (!empty($startDate))
         {
             if (!DateUtility::validate('-', $startDate, $dateFormatFlag))
@@ -1026,9 +1024,7 @@ private function onEdit()
          * convert the date to MySQL format.
          */
         $startDate = $this->getTrimmedInput('startDate', $_POST);
-        $dateFormatFlag = $_SESSION['CATS']->isDateDMY()
-            ? DATE_FORMAT_DDMMYY
-            : DATE_FORMAT_MMDDYY;
+        $dateFormatFlag = $_SESSION['CATS']->getDateFormat();
         if (!empty($startDate))
         {
             if (!DateUtility::validate('-', $startDate, $dateFormatFlag))
diff --git a/modules/login/wizard/Localization.tpl b/modules/login/wizard/Localization.tpl
index d8d650aab..ad3da665b 100755
--- a/modules/login/wizard/Localization.tpl
+++ b/modules/login/wizard/Localization.tpl
@@ -11,8 +11,9 @@
         <td style="font-size: 14px;">Date Format</td>
         <td style="font-size: 14px; padding-bottom: 5px;">
             <select id="dateFormat" name="dateFormat" style="width: 150px;">
-                <option value="mdy"<?php if (!$this->isDateDMY): ?> selected<?php endif; ?>>MM-DD-YYYY (US)</option>
-                <option value="dmy"<?php if ($this->isDateDMY): ?> selected<?php endif; ?>>DD-MM-YYYY (UK)</option>
+                <option value="mdy"<?php if ($this->dateFormat == DATE_FORMAT_MMDDYY): ?> selected<?php endif; ?>>MM-DD-YYYY (US)</option>
+                <option value="dmy"<?php if ($this->dateFormat == DATE_FORMAT_DDMMYY): ?> selected<?php endif; ?>>DD-MM-YYYY (UK)</option>
+                <option value="ymd"<?php if ($this->dateFormat == DATE_FORMAT_YYYYMMDD): ?> selected<?php endif; ?>>YYYY-MM-DD (ISO)</option>
             </select>
         </td>
     </tr>
diff --git a/modules/reports/ReportsUI.php b/modules/reports/ReportsUI.php
index 6fdc296e5..ade0e12b6 100755
--- a/modules/reports/ReportsUI.php
+++ b/modules/reports/ReportsUI.php
@@ -397,7 +397,7 @@ private function customizeJobOrderReport()
         $reportParameters['periodLine'] = sprintf(
             '%s - %s',
             strtok($data['dateCreated'], ' '),
-            DateUtility::getAdjustedDate('m-d-y')
+            DateUtility::getAdjustedDate(DateUtility::getPhpDateFormat())
         );
 
         $reportParameters['dataSet1'] = $data['pipeline'];
diff --git a/modules/settings/Localization.tpl b/modules/settings/Localization.tpl
index d3f62685d..4cb51f5f9 100755
--- a/modules/settings/Localization.tpl
+++ b/modules/settings/Localization.tpl
@@ -34,8 +34,9 @@
                                 <div class="row g-2 mb-2">
                                     <div class="col-12 col-sm">
                                         <select id="dateFormat" name="dateFormat" class="form-select form-select-sm">
-                                            <option value="mdy"<?php if (!$this->isDateDMY): ?> selected<?php endif; ?>>MM-DD-YYYY (US)</option>
-                                            <option value="dmy"<?php if ($this->isDateDMY): ?> selected<?php endif; ?>>DD-MM-YYYY (UK)</option>
+                                            <option value="mdy"<?php if ($this->dateFormat == DATE_FORMAT_MMDDYY): ?> selected<?php endif; ?>>MM-DD-YYYY (US)</option>
+                                            <option value="dmy"<?php if ($this->dateFormat == DATE_FORMAT_DDMMYY): ?> selected<?php endif; ?>>DD-MM-YYYY (UK)</option>
+                                            <option value="ymd"<?php if ($this->dateFormat == DATE_FORMAT_YYYYMMDD): ?> selected<?php endif; ?>>YYYY-MM-DD (ISO)</option>
                                         </select>
                                     </div>
                                 </div>
diff --git a/modules/settings/NewInstallWizard.tpl b/modules/settings/NewInstallWizard.tpl
index 9ad5f5f38..30a0af11f 100755
--- a/modules/settings/NewInstallWizard.tpl
+++ b/modules/settings/NewInstallWizard.tpl
@@ -83,6 +83,7 @@
                                     <select id="dateFormat" name="dateFormat" class="form-select form-select-sm">
                                         <option value="mdy" selected="selected">MM-DD-YYYY (US)</option>
                                         <option value="dmy">DD-MM-YYYY (UK)</option>
+                                        <option value="ymd">YYYY-MM-DD (ISO)</option>
                                     </select>
                                 </div>
                             </div>
diff --git a/modules/settings/SettingsUI.php b/modules/settings/SettingsUI.php
index 272bb98f8..8d002c6ec 100755
--- a/modules/settings/SettingsUI.php
+++ b/modules/settings/SettingsUI.php
@@ -2414,7 +2414,7 @@ private function administration()
                     }
 
                     $this->_template->assign('timeZone', $_SESSION['CATS']->getTimeZone());
-                    $this->_template->assign('isDateDMY', $_SESSION['CATS']->isDateDMY());
+                    $this->_template->assign('dateFormat', $_SESSION['CATS']->getDateFormat());
                     $this->_template->assign('isTimeFormat24', $_SESSION['CATS']->isTimeFormat24());
 
                     // Default phone country calling code for the localization settings page.
@@ -2564,20 +2564,12 @@ private function onAdministration()
                 }
                 //FIXME: Validation (escaped at lib level anyway)
                 $timeZone = $_POST['timeZone'];
-                $dateFormat = $_POST['dateFormat'];
-                if ($dateFormat == 'mdy')
-                {
-                    $isDMY = false;
-                }
-                else
-                {
-                    $isDMY = true;
-                }
+                $dateFormat = DateUtility::getDateFormatFromFormValue($_POST['dateFormat']);
 
                 $isTimeFormat24 = (isset($_POST['timeFormat']) && $_POST['timeFormat'] === '24');
 
                 $site = new Site();
-                $site->setLocalization($timeZone, $isDMY, $isTimeFormat24);
+                $site->setLocalization($timeZone, $dateFormat, $isTimeFormat24);
 
                 // Default phone country calling code (E.164) for the site.
                 if (isset($_POST['defaultPhoneCountryCodeDigits']))
@@ -2620,23 +2612,15 @@ private function onAspLocalization()
         // FIXME: Input validation!
 
         $timeZone = $_POST['timeZone'];
-        $dateFormat = $_POST['dateFormat'];
-        if ($dateFormat == 'mdy')
-        {
-            $isDMY = false;
-        }
-        else
-        {
-            $isDMY = true;
-        }
+        $dateFormat = DateUtility::getDateFormatFromFormValue($_POST['dateFormat']);
 
         $isTimeFormat24 = (isset($_POST['timeFormat']) && $_POST['timeFormat'] === '24');
 
         $site = new Site();
-        $site->setLocalization($timeZone, $isDMY, $isTimeFormat24);
+        $site->setLocalization($timeZone, $dateFormat, $isTimeFormat24);
 
         /* Reload the new data for the session. */
-        $_SESSION['CATS']->setTimeDateLocalization($timeZone, $isDMY, $isTimeFormat24);
+        $_SESSION['CATS']->setTimeDateLocalization($timeZone, $dateFormat, $isTimeFormat24);
 
         $this->_template->assign('inputType', 'conclusion');
         $this->_template->assign('title', 'Localization Settings Saved!');
@@ -3042,20 +3026,12 @@ private function wizard_localization()
         }
 
         $timeZone = $_GET['timeZone'];
-        $dateFormat = $_GET['dateFormat'];
-        if ($dateFormat == 'mdy')
-        {
-            $isDMY = false;
-        }
-        else
-        {
-            $isDMY = true;
-        }
+        $dateFormat = DateUtility::getDateFormatFromFormValue($_GET['dateFormat']);
 
         $isTimeFormat24 = (isset($_GET['timeFormat']) && $_GET['timeFormat'] === '24');
 
         $site = new Site();
-        $site->setLocalization($timeZone, $isDMY, $isTimeFormat24);
+        $site->setLocalization($timeZone, $dateFormat, $isTimeFormat24);
         $site->setLocalizationConfigured();
 
         echo 'Ok';
diff --git a/modules/wizard/WizardUI.php b/modules/wizard/WizardUI.php
index e4f787296..6f3409ee1 100755
--- a/modules/wizard/WizardUI.php
+++ b/modules/wizard/WizardUI.php
@@ -44,7 +44,7 @@ public function __construct()
         ');
         $this->addPage('Localization', './modules/wizard/WizardIntroLocalization.tpl', '
             $this->_template->assign(\'timeZone\', $_SESSION[\'CATS\']->getTimeZone());
-            $this->_template->assign(\'isDateDMY\', $_SESSION[\'CATS\']->isDateDMY());
+            $this->_template->assign(\'dateFormat\', $_SESSION[\'CATS\']->getDateFormat());
             $this->_template->assign(\'isTimeFormat24\', $_SESSION[\'CATS\']->isTimeFormat24());
         ');
 
diff --git a/src/OpenCATS/Tests/IntegrationTests/ChartDataTest.php b/src/OpenCATS/Tests/IntegrationTests/ChartDataTest.php
index 31015bb2e..8a5662c3f 100644
--- a/src/OpenCATS/Tests/IntegrationTests/ChartDataTest.php
+++ b/src/OpenCATS/Tests/IntegrationTests/ChartDataTest.php
@@ -11,6 +11,7 @@ public function testExistingQueriesAndChartDataAcrossRangesAndFilters()
             public function getUserID() { return 1; }
             public function isLoggedIn() { return false; }
             public function isDateDMY() { return true; }
+            public function getDateFormat() { return DATE_FORMAT_DDMMYY; }
             public function isTimeFormat24() { return true; }
             public function getTimeZoneOffset() { return 0; }
         };
diff --git a/src/OpenCATS/Tests/IntegrationTests/EntityDeleteCleanupTest.php b/src/OpenCATS/Tests/IntegrationTests/EntityDeleteCleanupTest.php
index a612e3b4e..441a2dd98 100644
--- a/src/OpenCATS/Tests/IntegrationTests/EntityDeleteCleanupTest.php
+++ b/src/OpenCATS/Tests/IntegrationTests/EntityDeleteCleanupTest.php
@@ -43,6 +43,11 @@ public function isDateDMY()
                 return false;
             }
 
+            public function getDateFormat()
+            {
+                return DATE_FORMAT_MMDDYY;
+            }
+
             public function isTimeFormat24()
             {
                 return false;
diff --git a/src/OpenCATS/Tests/UnitTests/DateUtilityTest.php b/src/OpenCATS/Tests/UnitTests/DateUtilityTest.php
index aaf6bf8b6..c9d0656a1 100644
--- a/src/OpenCATS/Tests/UnitTests/DateUtilityTest.php
+++ b/src/OpenCATS/Tests/UnitTests/DateUtilityTest.php
@@ -242,6 +242,47 @@ function testGetDateTimeFormat()
         $this->assertSame('d-m-Y H:i',   DateUtility::getDateTimeFormat('d-m-Y', true));
     }
 
+    /* Tests for date format flag mapping. */
+    function testDateFormatMapping()
+    {
+        $formats = array(
+            /* site value, form value, flag, JS format, PHP format */
+            array(0, 'mdy', DATE_FORMAT_MMDDYY,   'MM-DD-YY',   'm-d-y'),
+            array(1, 'dmy', DATE_FORMAT_DDMMYY,   'DD-MM-YY',   'd-m-y'),
+            array(2, 'ymd', DATE_FORMAT_YYYYMMDD, 'YYYY-MM-DD', 'Y-m-d'),
+        );
+
+        foreach ($formats as $value)
+        {
+            $this->assertSame($value[2], DateUtility::getDateFormatFromSiteValue($value[0]));
+            $this->assertSame($value[0], DateUtility::getSiteValueFromDateFormat($value[2]));
+            $this->assertSame($value[2], DateUtility::getDateFormatFromFormValue($value[1]));
+            $this->assertSame($value[3], DateUtility::getJsDateFormat($value[2]));
+            $this->assertSame($value[4], DateUtility::getPhpDateFormat($value[2]));
+        }
+
+        /* Unknown values fall back to M-D-Y. */
+        $this->assertSame(DATE_FORMAT_MMDDYY, DateUtility::getDateFormatFromSiteValue(null));
+        $this->assertSame(DATE_FORMAT_MMDDYY, DateUtility::getDateFormatFromFormValue('invalid'));
+
+        /* Legacy boolean D-M-Y values are accepted. */
+        $this->assertSame(DATE_FORMAT_DDMMYY, DateUtility::normalizeDateFormat(true));
+        $this->assertSame(DATE_FORMAT_MMDDYY, DateUtility::normalizeDateFormat(false));
+        $this->assertSame(DATE_FORMAT_YYYYMMDD, DateUtility::normalizeDateFormat(DATE_FORMAT_YYYYMMDD));
+    }
+
+    /* Tests for formatStoredDate(). */
+    function testFormatStoredDate()
+    {
+        $this->assertSame('12-31-05',   DateUtility::formatStoredDate('12-31-05', DATE_FORMAT_MMDDYY));
+        $this->assertSame('31-12-05',   DateUtility::formatStoredDate('12-31-05', DATE_FORMAT_DDMMYY));
+        $this->assertSame('2005-12-31', DateUtility::formatStoredDate('12-31-05', DATE_FORMAT_YYYYMMDD));
+
+        /* Empty and invalid values are returned unchanged. */
+        $this->assertSame('',        DateUtility::formatStoredDate('', DATE_FORMAT_YYYYMMDD));
+        $this->assertSame('invalid', DateUtility::formatStoredDate('invalid', DATE_FORMAT_DDMMYY));
+    }
+
     /* Tests for normalizeActivityTime(). */
     function testNormalizeActivityTime()
     {
diff --git a/src/OpenCATS/Tests/UnitTests/SessionDateFormatTest.php b/src/OpenCATS/Tests/UnitTests/SessionDateFormatTest.php
new file mode 100644
index 000000000..a8e157bb8
--- /dev/null
+++ b/src/OpenCATS/Tests/UnitTests/SessionDateFormatTest.php
@@ -0,0 +1,43 @@
+<?php
+use PHPUnit\Framework\TestCase;
+
+include_once(LEGACY_ROOT . '/constants.php');
+include_once(LEGACY_ROOT . '/lib/Session.php');
+
+class SessionDateFormatTest extends TestCase
+{
+    function testSetTimeDateLocalizationAcceptsLegacyBoolean()
+    {
+        $session = new CATSSession();
+
+        $session->setTimeDateLocalization(0, true);
+        $this->assertSame(DATE_FORMAT_DDMMYY, $session->getDateFormat());
+        $this->assertTrue($session->isDateDMY());
+
+        $session->setTimeDateLocalization(0, false);
+        $this->assertSame(DATE_FORMAT_MMDDYY, $session->getDateFormat());
+
+        $session->setTimeDateLocalization(0, DATE_FORMAT_YYYYMMDD);
+        $this->assertSame(DATE_FORMAT_YYYYMMDD, $session->getDateFormat());
+    }
+
+    /* A session serialized before the upgrade stores a boolean _dateDMY;
+     * _dateFormat then keeps its M-D-Y default until __wakeup() runs. */
+    function testLegacySessionKeepsDateFormat()
+    {
+        foreach (array(1 => DATE_FORMAT_DDMMYY, 0 => DATE_FORMAT_MMDDYY) as $isDMY => $expected)
+        {
+            $serialized = str_replace(
+                '_dateDMY";N;',
+                '_dateDMY";b:' . $isDMY . ';',
+                serialize(new CATSSession())
+            );
+            $this->assertStringContainsString('_dateDMY";b:', $serialized);
+            $session = unserialize($serialized);
+
+            $this->assertSame($expected, $session->getDateFormat());
+            $this->assertStringNotContainsString('_dateDMY";b:', serialize($session));
+        }
+    }
+}
+?>
diff --git a/test/features/activities.feature b/test/features/activities.feature
index 55b455937..c7791d193 100644
--- a/test/features/activities.feature
+++ b/test/features/activities.feature
@@ -152,9 +152,10 @@ Feature: Activities
     Then I should see "Activities combined fixture"
 
     Examples:
-      | format | clock | date     | hour |
-      | MDY    | 12    | 06-12-31 | 9    |
-      | DMY    | 24    | 12-06-31 | 21   |
+      | format | clock | date       | hour |
+      | MDY    | 12    | 06-12-31   | 9    |
+      | DMY    | 24    | 12-06-31   | 21   |
+      | YMD    | 24    | 2031-06-12 | 21   |
 
   @javascript @activities_form
   Scenario: Schedule an all-day private event without logging an activity
diff --git a/test/features/bootstrap/ActivitiesSteps.php b/test/features/bootstrap/ActivitiesSteps.php
index df347cd36..9e0276e59 100644
--- a/test/features/bootstrap/ActivitiesSteps.php
+++ b/test/features/bootstrap/ActivitiesSteps.php
@@ -43,7 +43,7 @@ public function activitiesFormats($dateFormat, $timeFormat)
     {
         $db = DatabaseConnection::getInstance();
         $this->activitiesFormats = $db->getAllAssoc('SELECT site_id, date_format_ddmmyy, time_format_24 FROM site');
-        $db->query('UPDATE site SET date_format_ddmmyy = ' . ($dateFormat === 'DMY' ? 1 : 0)
+        $db->query('UPDATE site SET date_format_ddmmyy = ' . array('MDY' => 0, 'DMY' => 1, 'YMD' => 2)[$dateFormat]
             . ', time_format_24 = ' . ($timeFormat === '24' ? 1 : 0));
     }
 
diff --git a/test/features/bootstrap/FeatureContext.php b/test/features/bootstrap/FeatureContext.php
index ae4a82898..655734b29 100644
--- a/test/features/bootstrap/FeatureContext.php
+++ b/test/features/bootstrap/FeatureContext.php
@@ -151,7 +151,7 @@ public function calendarUsesFormats($dateFormat, $timeFormat)
         $this->calendarSiteFormats = $db->getAllAssoc(
             'SELECT site_id, date_format_ddmmyy, time_format_24 FROM site'
         );
-        $db->query('UPDATE site SET date_format_ddmmyy = ' . ($dateFormat === 'DMY' ? 1 : 0)
+        $db->query('UPDATE site SET date_format_ddmmyy = ' . array('MDY' => 0, 'DMY' => 1, 'YMD' => 2)[$dateFormat]
             . ', time_format_24 = ' . ($timeFormat === '24' ? 1 : 0));
     }
 
diff --git a/test/features/calendar.feature b/test/features/calendar.feature
index 2176023d2..55b8cb8f6 100644
--- a/test/features/calendar.feature
+++ b/test/features/calendar.feature
@@ -22,7 +22,7 @@ Feature: Calendar
     And I select "Meeting" from "type"
     And I select "Jun" from "dateAdd_Month_ID"
     And I select "12" from "dateAdd_Day_ID"
-    And I fill in "dateAdd_Year_ID" with "30"
+    And I fill in "dateAdd_Year_ID" with "<year>"
     And I select "09" from "hour"
     And I select "15" from "minute"
     And I fill in "description" with "Calendar workflow details"
@@ -53,7 +53,7 @@ Feature: Calendar
     Then the "#calendarMonth" element should contain "Calendar workflow event"
     When I click on the element "#calendarMonth .calendarEntry"
     And I press "Edit Event"
-    Then the "dateEdit_Year_ID" field should contain "30"
+    Then the "dateEdit_Year_ID" field should contain "<year>"
     And the "minuteEdit" field should contain "15"
     And Calendar panel "sendEmailTDEdit" should be "visible"
     When I uncheck "reminderToggleEdit"
@@ -79,9 +79,10 @@ Feature: Calendar
     Then I should see "My Upcoming Events / Calls"
 
     Examples:
-      | dateFormat | timeFormat |
-      | MDY        | 12         |
-      | DMY        | 24         |
+      | dateFormat | timeFormat | year |
+      | MDY        | 12         | 30   |
+      | DMY        | 24         | 30   |
+      | YMD        | 24         | 2030 |
 
   Scenario Outline: Navigate beyond cached months and select a date to create an event
     Given Calendar AJAX is "<ajax>"