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,