timetable performance optimization

test/extensions/date_time_test.dart

@@ -86,6 +86,15 @@ void main() {
       final d = DateTime(2026, 5, 18, 13, 45);
       expect(d.subtractDays(5), d.addDays(-5));
     });
+
+    test('dateOnly drops the time of day', () {
+      expect(DateTime(2026, 9, 24, 13, 45).dateOnly, DateTime(2026, 9, 24));
+    });
+
+    test('mondayOfWeek returns Monday midnight for any weekday', () {
+      expect(DateTime(2026, 9, 21).mondayOfWeek, DateTime(2026, 9, 21));
+      expect(DateTime(2026, 9, 27, 23, 59).mondayOfWeek, DateTime(2026, 9, 21));
+    });
   });
 
   group('DateTimeFormatting', () {