timetable performance optimization

lib/extensions/date_time.dart

@@ -36,6 +36,11 @@ extension CalendarDayArithmetic on DateTime {
       );
 
   DateTime subtractDays(int days) => addDays(-days);
+
+  DateTime get dateOnly => DateTime(year, month, day);
+
+  /// Monday 00:00 of this date's week.
+  DateTime get mondayOfWeek => subtractDays(weekday - 1).dateOnly;
 }
 
 /// Formatting helpers backed by Jiffy. Centralises the patterns that previously