diff --git a/calendar/js/app.js b/calendar/js/app.js
index 59a22a23dc..e4e56b5780 100644
--- a/calendar/js/app.js
+++ b/calendar/js/app.js
@@ -3571,8 +3571,8 @@ var CalendarApp = /** @class */ (function (_super) {
             },
             // Dates are user time, but we told javascript it was UTC.
             // Send just the timestamp (as a string) with no timezone
-            _data.start.toJSON().slice(0, -1),
-            _data.end.toJSON().slice(0, -1),
+            (typeof _data.start != "string" ? _data.start.toJSON() : _data.start).slice(0, -1),
+            (typeof _data.end != "string" ? _data.end.toJSON() : _data.end).slice(0, -1),
             {
                 participants: Object.keys(_data.participants).filter(function (v) { return v.match(/^[0-9]/); })
             }], function (_value) {
diff --git a/calendar/js/app.ts b/calendar/js/app.ts
index fe7221f32c..40c64d3b91 100644
--- a/calendar/js/app.ts
+++ b/calendar/js/app.ts
@@ -4432,8 +4432,8 @@ class CalendarApp extends EgwApp
 				},
 			    // Dates are user time, but we told javascript it was UTC.
 			    // Send just the timestamp (as a string) with no timezone
-                _data.start.toJSON().slice(0,-1),
-                _data.end.toJSON().slice(0,-1),
+				( typeof _data.start != "string" ? _data.start.toJSON() : _data.start).slice(0,-1),
+                ( typeof _data.end != "string" ? _data.end.toJSON() : _data.end).slice(0,-1),
 				{
 					participants:Object.keys(_data.participants).filter(v => {return v.match(/^[0-9]/)})
 				}], function(_value){