diff --git a/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php b/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php
index 38e044b3d6..b79a560422 100644
--- a/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php
+++ b/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php
@@ -1,69 +1,77 @@
 <?php
 
 abstract class PhabricatorCalendarEventDateTransaction
   extends PhabricatorCalendarEventTransactionType {
 
   abstract protected function getInvalidDateMessage();
 
   public function isInheritedEdit() {
     return false;
   }
 
   public function generateNewValue($object, $value) {
     $editor = $this->getEditor();
 
     if ($value->isDisabled()) {
       return null;
     }
 
     return $value->newPhutilDateTime()
       ->setIsAllDay($editor->getNewIsAllDay())
       ->newAbsoluteDateTime()
       ->toDictionary();
   }
 
   public function getTransactionHasEffect($object, $old, $new) {
+    // If either value is `null` (for example, when setting a recurring event
+    // end date for the first time) and the other value is not `null`, this
+    // transaction has an effect.
+    $has_null = (($old === null) || ($new === null));
+    if ($has_null) {
+      return ($old !== $new);
+    }
+
     $editor = $this->getEditor();
 
     $actor = $this->getActor();
     $actor_timezone = $actor->getTimezoneIdentifier();
 
     // When an edit only changes the timezone of an event without materially
     // changing the absolute time, discard it. This can happen if two users in
     // different timezones edit an event without rescheduling it.
 
     // Eventually, after T11073, there may be a UI control to adjust timezones.
     // If a user explicitly changed the timezone, we should respect that.
     // However, there is no way for users to intentionally apply this kind of
     // edit today.
 
     $old_datetime = PhutilCalendarAbsoluteDateTime::newFromDictionary($old)
       ->setIsAllDay($editor->getNewIsAllDay())
       ->setViewerTimezone($actor_timezone);
 
     $new_datetime = PhutilCalendarAbsoluteDateTime::newFromDictionary($new)
       ->setIsAllDay($editor->getNewIsAllDay())
       ->setViewerTimezone($actor_timezone);
 
     $old_epoch = $old_datetime->getEpoch();
     $new_epoch = $new_datetime->getEpoch();
 
     return ($old_epoch !== $new_epoch);
   }
 
   public function validateTransactions($object, array $xactions) {
     $errors = array();
 
     foreach ($xactions as $xaction) {
       if ($xaction->getNewValue()->isValid()) {
         continue;
       }
 
       $message = $this->getInvalidDateMessage();
       $errors[] = $this->newInvalidError($message, $xaction);
     }
 
     return $errors;
   }
 
 }