Umstellung auf UTC Timestamp

lib/content_views/main_page.dart

@@ -420,7 +420,7 @@ class _MainPageState extends State<MainPage> with ConsumerMixin, ProducerMixin {
     var logoutMessage = LogoutMessage();
     if (position != null) {
       logoutMessage.pos = LogoutMessagePosition();
-      logoutMessage.pos!.time = position.timestamp.toIso8601String();
+      logoutMessage.pos!.time = Util.toUnixMillisUtc(position.timestamp);
       logoutMessage.pos!.lat = position.latitude;
       logoutMessage.pos!.long = position.longitude;
       logoutMessage.pos!.acc = position.accuracy.toInt();