diff --git a/web/edit_entry.php b/web/edit_entry.php
index e2a516790..e4d9fb7a8 100644
--- a/web/edit_entry.php
+++ b/web/edit_entry.php
@@ -1183,7 +1183,7 @@ function getbookingdate(int $t, bool $is_end=false) : array
     // The end date that came through from the drag select is actually the repeat end
     // date, and the real end date will actually be the start date.
     $rep_type = RepeatRule::DAILY;
-    $rep_end_date = DateTime::createFromFormat('Y-m-d', $end_date);
+    $rep_end_date = DateTime::createFromFormat(DateTime::ISO8601_DATE, $end_date);
     $end_date = $start_date;
   }
 }
diff --git a/web/edit_entry_handler.php b/web/edit_entry_handler.php
index 25eb805c1..da629db29 100644
--- a/web/edit_entry_handler.php
+++ b/web/edit_entry_handler.php
@@ -628,7 +628,7 @@ function invalid_booking(string $message) : void
   }
   if (isset($rep_end_date))
   {
-    $repeat_end_date = DateTime::createFromFormat('Y-m-d', $rep_end_date);
+    $repeat_end_date = DateTime::createFromFormat(DateTime::ISO8601_DATE, $rep_end_date);
     if ($repeat_end_date === false)
     {
       throw new Exception("Could not create repeat end date");
diff --git a/web/functions.inc b/web/functions.inc
index 153df24b7..28ea5aaa6 100644
--- a/web/functions.inc
+++ b/web/functions.inc
@@ -1766,7 +1766,7 @@ function times_somewhere(bool $only_enabled=false) : bool
 function format_iso_date(int $year, int $month, int $day) : string
 {
   $date = new DateTime();
-  return $date->setDate($year, $month, $day)->format('Y-m-d');
+  return $date->setDate($year, $month, $day)->format(DateTime::ISO8601_DATE);
 }
 
 
@@ -1775,7 +1775,7 @@ function format_iso_date(int $year, int $month, int $day) : string
 // on failure.
 function split_iso_date(string $iso_date)
 {
-  if (false === ($date = DateTime::createFromFormat('Y-m-d', $iso_date)))
+  if (false === ($date = DateTime::createFromFormat(DateTime::ISO8601_DATE, $iso_date)))
   {
     return false;
   }
@@ -1795,7 +1795,7 @@ function validate_iso_date(string $date) : bool
   // the inverse operation gives the original string then it is
   // valid.  (This is to check that, for example, 2021-09-31 hasn't
   // been converted into 2021-10-01).
-  $format = 'Y-m-d';
+  $format = DateTime::ISO8601_DATE;
   $datetime = DateTime::createFromFormat($format, $date);
 
   return ($datetime && ($datetime->format($format) === $date));
diff --git a/web/functions_view.inc b/web/functions_view.inc
index 33f76fe55..68a831396 100644
--- a/web/functions_view.inc
+++ b/web/functions_view.inc
@@ -134,7 +134,7 @@ function create_details_body(array $data, bool $as_html=false, bool $keep_privat
             // Convert DATE types into a locale friendly form
             if (isset($column) && ('date' == $column->getType()))
             {
-              $date = DateTime::createFromFormat('Y-m-d', $data[$key]);
+              $date = DateTime::createFromFormat(DateTime::ISO8601_DATE, $data[$key]);
               if ($date !== false)
               {
                 $data[$key] = datetime_format($datetime_formats['date'],  $date->getTimestamp());
diff --git a/web/lib/MRBS/DateTime.php b/web/lib/MRBS/DateTime.php
index 021d23e19..69acc38b0 100644
--- a/web/lib/MRBS/DateTime.php
+++ b/web/lib/MRBS/DateTime.php
@@ -11,6 +11,7 @@ class DateTime extends \DateTime
   private static $isHoliday = array();
   private static $validHolidays = array();
 
+  public const ISO8601_DATE = 'Y-m-d';
   private const HOLIDAY_RANGE_OPERATOR = '..';
 
 
@@ -187,7 +188,7 @@ public function getYear() : int
   // Returns a date in ISO 8601 format ('yyyy-mm-dd')
   public function getISODate() : string
   {
-    return $this->format('Y-m-d');
+    return $this->format(self::ISO8601_DATE);
   }
 
 
@@ -329,7 +330,7 @@ private function isHolidayConfig() : bool
     global $holidays;
 
     $year = $this->format('Y');
-    $iso_date = $this->format('Y-m-d');
+    $iso_date = $this->getISODate();
 
     // Only need to check if a date is a holiday once, so store the answer in a
     // static property
@@ -398,7 +399,7 @@ public function isHoliday() : bool
   public function isToday() : bool
   {
     $today = new DateTime();
-    return ($this->format('Y-m-d') == $today->format('Y-m-d'));
+    return ($this->getISODate() == $today->getISODate());
   }
 
 
diff --git a/web/lib/MRBS/EntryInterval.php b/web/lib/MRBS/EntryInterval.php
index 2ed8c6d83..36e49ec7d 100644
--- a/web/lib/MRBS/EntryInterval.php
+++ b/web/lib/MRBS/EntryInterval.php
@@ -166,8 +166,7 @@ public function overlapsWeekend()
   // Checks whether an entry spans more than one calendar (not booking) day
   public function spansMultipleDays() : bool
   {
-    $format = 'Y-m-d';
-    return ($this->start_date->format($format) !== $this->end_date->format($format));
+    return ($this->start_date->getISODate() !== $this->end_date->getISODate());
   }
 
 
diff --git a/web/mrbs_sql.inc b/web/mrbs_sql.inc
index 1b24813eb..7bda90066 100644
--- a/web/mrbs_sql.inc
+++ b/web/mrbs_sql.inc
@@ -153,7 +153,7 @@ function mrbsCheckFree(array $booking, ?int $ignore_entry_id=null, ?int $ignore_
     }
 
     // enclose the viewday etc. links in a span to make it easier for JavaScript to strip them out
-    $vars = array('page_date' => $existing_start_date->format('Y-m-d'),
+    $vars = array('page_date' => $existing_start_date->getISODate(),
                   'area'  => $area,
                   'room'  => $room_id);
 
@@ -602,7 +602,7 @@ function mrbsCheckPolicy(array $booking, ?int $ignore_entry_id=null, ?int $ignor
   if ($min_booking_date_enabled && !$delete)
   {
     // Use DateTimeImmutable because we will need the original date for the error message
-    $date_immutable = DateTimeImmutable::createFromFormat($format='Y-m-d', $min_booking_date);
+    $date_immutable = DateTimeImmutable::createFromFormat($format=DateTime::ISO8601_DATE, $min_booking_date);
     // Check that the date is valid (createFromFormat will allow, for example, months > 12)
     if (($date_immutable === false) || ($date_immutable->format($format) !== $min_booking_date))
     {
@@ -621,7 +621,7 @@ function mrbsCheckPolicy(array $booking, ?int $ignore_entry_id=null, ?int $ignor
   if ($max_booking_date_enabled && !$delete)
   {
     // Use DateTimeImmutable because we will need the original date for the error message
-    $date_immutable = DateTimeImmutable::createFromFormat($format='Y-m-d', $max_booking_date);
+    $date_immutable = DateTimeImmutable::createFromFormat($format=DateTime::ISO8601_DATE, $max_booking_date);
     // Check that the date is valid (createFromFormat will allow, for example, months > 12)
     if (($date_immutable === false) || ($date_immutable->format($format) !== $max_booking_date))
     {
diff --git a/web/report.php b/web/report.php
index 2dbf90844..2661ce9f9 100644
--- a/web/report.php
+++ b/web/report.php
@@ -1477,8 +1477,8 @@ function get_match_condition(string $full_column_name, ?string $match, array &$s
 
 $default_from_time = mktime(0, 0, 0, $month, $day, $year);
 $default_to_time = mktime(0, 0, 0, $month, $day + $default_report_days, $year);
-$default_from_date = date('Y-m-d', $default_from_time);
-$default_to_date = date('Y-m-d', $default_to_time);
+$default_from_date = date(DateTime::ISO8601_DATE, $default_from_time);
+$default_to_date = date(DateTime::ISO8601_DATE, $default_to_time);
 
 // Get non-standard form variables
 $from_date = get_form_var('from_date', 'string', $default_from_date);
@@ -1632,16 +1632,16 @@ function get_match_condition(string $full_column_name, ?string $match, array &$s
 if ($phase == 2)
 {
   // Start and end times are also used to clip the times for summary info.
-  // createFromFormat('Y-m-d') gives the current time.  We want the report to
+  // createFromFormat() gives the current time.  We want the report to
   // start at the beginning of the start day and end of the day, so set the
   // times accordingly.
-  if (false === ($report_start = DateTime::createFromFormat('Y-m-d', $from_date)))
+  if (false === ($report_start = DateTime::createFromFormat(DateTime::ISO8601_DATE, $from_date)))
   {
     throw new Exception("Invalid from_date '$from_date'");
   }
   $report_start = $report_start->setTime(0, 0)->getTimestamp();
 
-  if (false === ($report_end = DateTime::createFromFormat('Y-m-d', $to_date)))
+  if (false === ($report_end = DateTime::createFromFormat(DateTime::ISO8601_DATE, $to_date)))
   {
     throw new Exception("Invalid to_date '$to_date'");
   }
diff --git a/web/search.php b/web/search.php
index 49e886b3e..3ea3825d4 100644
--- a/web/search.php
+++ b/web/search.php
@@ -158,7 +158,7 @@ function output_row($row, $returl)
 {
   if (validate_iso_date($from_date))
   {
-    $search_start_time = DateTime::createFromFormat('Y-m-d', $from_date)->setTime(0, 0)->getTimestamp();
+    $search_start_time = DateTime::createFromFormat(DateTime::ISO8601_DATE, $from_date)->setTime(0, 0)->getTimestamp();
   }
   else
   {