#10 Fixed gray background in timetable on days outside of given range

lib/extensions/dateTime.dart

@@ -2,4 +2,8 @@ extension IsSameDay on DateTime {
   bool isSameDay(DateTime other) {
     return year == other.year && month == other.month && day == other.day;
   }
+
+  DateTime nextWeekday(int day) {
+    return add(Duration(days: (day - weekday) % DateTime.daysPerWeek));
+  }
 }