diff --git a/api/js/jsapi/egw_timer.js b/api/js/jsapi/egw_timer.js
index 43dd1d69fd..4dafc0ede0 100644
--- a/api/js/jsapi/egw_timer.js
+++ b/api/js/jsapi/egw_timer.js
@@ -92,6 +92,7 @@ egw.extend('timer', egw.MODULE_GLOBAL, function()
 	 *
 	 * @param {string} _action
 	 * @param {string} _time
+	 * @throws string error-message
 	 */
 	function timerAction(_action, _time)
 	{
@@ -267,17 +268,22 @@ egw.extend('timer', egw.MODULE_GLOBAL, function()
 	 * @param object _timer
 	 * @param bool|undefined _pause true: pause, else: stop
 	 * @param string|Date|undefined _time stop-time, default current time
+	 * @throws string error-message when timer.start < _time
 	 */
 	function stopTimer(_timer, _pause, _time)
 	{
 		const time = _time ? new Date(_time) : new Date();
 		// update _timer state object
-		_timer.paused = _pause || false;
 		if (_timer.start)
 		{
+			if (time.valueOf() < _timer.start.valueOf())
+			{
+				throw egw.lang('Stop- or pause-time can not be before the start-time!');
+			}
 			_timer.offset = time.valueOf() - _timer.start.valueOf();
 			_timer.start = undefined;
 		}
+		_timer.paused = _pause || false;
 		// update timer display
 		updateTimer(timer, _timer);
 
@@ -334,10 +340,15 @@ egw.extend('timer', egw.MODULE_GLOBAL, function()
 					callback: (button_id, value) =>		// return false to prevent dialog closing
 					{
 						if (button_id !== 'close') {
-							timerAction(button_id.replace(/_([a-z]+)\[([a-z]+)\]/, '$1-$2'),
-								// eT2 operates in user-time, while timers here always operate in UTC
-								value.time ? new Date((new Date(value.time)).valueOf() + egw.getTimezoneOffset() * 60000) : undefined);
-							dialog._overlayContentNode.querySelector('et2-date-time').value = '';
+							try {
+								timerAction(button_id.replace(/_([a-z]+)\[([a-z]+)\]/, '$1-$2'),
+									// eT2 operates in user-time, while timers here always operate in UTC
+									value.time ? new Date((new Date(value.time)).valueOf() + egw.getTimezoneOffset() * 60000) : undefined);
+								dialog._overlayContentNode.querySelector('et2-date-time').value = '';
+							}
+							catch (e) {
+								Et2Dialog.alert(e, egw.lang('Invalid Input'), Et2Dialog.ERROR_MESSAGE);
+							}
 							setButtonState();
 							return false;
 						}
diff --git a/timesheet/lang/egw_de.lang b/timesheet/lang/egw_de.lang
index c78d0a2f0d..74a2453ccd 100644
--- a/timesheet/lang/egw_de.lang
+++ b/timesheet/lang/egw_de.lang
@@ -68,6 +68,7 @@ import options	timesheet	de	Import Optionen
 imports entries into the timesheet from a csv file.	timesheet	de	Importiert Einträge für den Stundenzettel aus einer CSV Datei.
 insert	timesheet	de	einfügen
 invalid field: %1 = %2, it needs to be a number.	timesheet	de	Ungültiges Feld: %1 =%2, es muss eine Zahl sein
+invalid input	common	de	Ungültige Eingabe
 invalid owner id: %1.  might be a bad field translation.  used %2 instead.	timesheet	de	Ungültiger Benutze ID: %1 könnte eine falsche Feldzuordnung haben. %2 wird stattdessen verwendet.
 last modified	timesheet	de	Zuletzt geändert
 last month	timesheet	de	Letzten Monat
@@ -135,6 +136,7 @@ status deleted.	timesheet	de	Status gelöscht
 status of created timesheets	timesheet	de	Status für neue Stundenzettel
 status updated.	timesheet	de	Status geändert
 stop	common	de	Stop
+stop- or pause-time can not be before the start-time!	common	de	Stop- oder Pause-Zeit kann nicht vor der Start-Zeit liegen!
 sum	timesheet	de	Summe
 sum %1:	timesheet	de	Summe %1:
 tag to mark positions for address labels	timesheet	de	Platzhalter, um die Position der Adresslabels festzulegen
diff --git a/timesheet/lang/egw_en.lang b/timesheet/lang/egw_en.lang
index bf7175369f..d9c1e06dfc 100644
--- a/timesheet/lang/egw_en.lang
+++ b/timesheet/lang/egw_en.lang
@@ -68,6 +68,7 @@ import options	timesheet	en	Import options
 imports entries into the timesheet from a csv file.	timesheet	en	Imports entries into the Time Sheet from a CSV file
 insert	timesheet	en	Insert
 invalid field: %1 = %2, it needs to be a number.	timesheet	en	Invalid field: %1 = %2, it needs to be a number.
+invalid input	common	en	Invalid input
 invalid owner id: %1.  might be a bad field translation.  used %2 instead.	timesheet	en	Invalid owner ID: %1. Might be a bad field translation. Used %2 instead.
 last modified	timesheet	en	Last modified
 last month	timesheet	en	Last month
@@ -135,6 +136,7 @@ status deleted.	timesheet	en	Status deleted.
 status of created timesheets	timesheet	en	Status of created timesheets
 status updated.	timesheet	en	Status updated.
 stop	common	en	Stop
+stop- or pause-time can not be before the start-time!	common	en	Stop- or pause-time can not be before the start-time!
 sum	timesheet	en	Sum
 sum %1:	timesheet	en	Sum %1:
 tag to mark positions for address labels	timesheet	en	Tag to mark positions for address labels