simplified code across the app and consolidated duplicated helpers

lib/view/pages/timetable/data/calendar_logic.dart

@@ -29,7 +29,7 @@ bool isOutsideSchoolHours(Appointment a) {
 }
 
 int dayIndex(DateTime t, DateTime weekStart) =>
-    DateTime(t.year, t.month, t.day).difference(weekStart).inDays;
+    t.dateOnly.difference(weekStart).inDays;
 
 class BoundRegion {
   final TimeRegion region;
@@ -41,7 +41,7 @@ class BoundRegion {
 
 List<BoundRegion> expandRegionsForDay(List<TimeRegion> regions, DateTime day) {
   final result = <BoundRegion>[];
-  final dayStart = DateTime(day.year, day.month, day.day);
+  final dayStart = day.dateOnly;
   for (final region in regions) {
     final isRecurringDaily =
         region.recurrenceRule != null &&
@@ -123,11 +123,7 @@ partitionAppointmentsForWeek(
         )) {
           continue;
         }
-        final idx = DateTime(
-          occLocal.year,
-          occLocal.month,
-          occLocal.day,
-        ).difference(weekStart).inDays;
+        final idx = dayIndex(occLocal, weekStart);
         if (idx < 0 || idx >= 5) continue;
         final newStart = DateTime(
           occLocal.year,

lib/view/pages/timetable/data/timetable_appointment_factory.dart

@@ -5,6 +5,7 @@ import '../../../../api/marianumconnect/models/mc_holiday.dart';
 import '../../../../api/marianumconnect/queries/timetable_get_subjects/timetable_get_subjects_response.dart';
 import '../../../../api/marianumconnect/queries/timetable_get_week/timetable_get_week_response.dart';
 import '../../../../api/mhsl/custom_timetable_event/custom_timetable_event.dart';
+import '../../../../extensions/date_time.dart';
 import '../../../../storage/timetable_settings.dart';
 import 'arbitrary_appointment.dart';
 import 'lesson_color.dart';
@@ -150,16 +151,8 @@ class TimetableAppointmentFactory {
   List<DateTime> _holidayExceptionDates(CustomTimetableEvent event) {
     final result = <DateTime>[];
     for (final holiday in holidays) {
-      var day = DateTime(
-        holiday.startDate.year,
-        holiday.startDate.month,
-        holiday.startDate.day,
-      );
-      final last = DateTime(
-        holiday.endDate.year,
-        holiday.endDate.month,
-        holiday.endDate.day,
-      );
+      var day = holiday.startDate.dateOnly;
+      final last = holiday.endDate.dateOnly;
       while (!day.isAfter(last)) {
         result.add(
           DateTime(
@@ -171,7 +164,7 @@ class TimetableAppointmentFactory {
             event.startDate.second,
           ),
         );
-        day = DateTime(day.year, day.month, day.day + 1);
+        day = day.addDays(1);
       }
     }
     return result;