diff --git a/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php b/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php
--- a/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php
+++ b/src/applications/calendar/xaction/PhabricatorCalendarEventDateTransaction.php
@@ -22,6 +22,35 @@
       ->toDictionary();
   }
 
+  public function getTransactionHasEffect($object, $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();
 
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -502,6 +502,15 @@
         return false;
     }
 
+    $type = $xaction->getTransactionType();
+    $xtype = $this->getModularTransactionType($type);
+    if ($xtype) {
+      return $xtype->getTransactionHasEffect(
+        $object,
+        $xaction->getOldValue(),
+        $xaction->getNewValue());
+    }
+
     return ($xaction->getOldValue() !== $xaction->getNewValue());
   }
 
diff --git a/src/applications/transactions/storage/PhabricatorModularTransactionType.php b/src/applications/transactions/storage/PhabricatorModularTransactionType.php
--- a/src/applications/transactions/storage/PhabricatorModularTransactionType.php
+++ b/src/applications/transactions/storage/PhabricatorModularTransactionType.php
@@ -35,6 +35,10 @@
     return;
   }
 
+  public function getTransactionHasEffect($object, $old, $new) {
+    return ($old !== $new);
+  }
+
   public function extractFilePHIDs($object, $value) {
     return array();
   }