improved performance and battery usage on older devices

lib/api/marianumconnect/queries/timetable_get_week/timetable_get_week_response.dart

@@ -76,10 +76,17 @@ class McTimetableEntry {
 
   /// Combines the calendar date with the hour/minute portion of [startTime]
   /// (which carries a 1970 placeholder date) into a real DateTime.
-  DateTime get startDateTime =>
+  DateTime get startDateTime => _startDateTime;
+
+  DateTime get endDateTime => _endDateTime;
+
+  // Cached (private, so json_serializable ignores them): sorting and lesson
+  // merging read these per comparison, and every local DateTime construction
+  // does a timezone lookup.
+  late final DateTime _startDateTime =
       DateTime(date.year, date.month, date.day, startTime.hour, startTime.minute);
 
-  DateTime get endDateTime =>
+  late final DateTime _endDateTime =
       DateTime(date.year, date.month, date.day, endTime.hour, endTime.minute);
 
   static DateTime _dateFromJson(String raw) => DateTime.parse(raw);