Added api for custom timetable events

lib/extensions/timeOfDay.dart

@@ -0,0 +1,15 @@
+import 'package:flutter/material.dart';
+
+extension TimeOfDayExt on TimeOfDay {
+  bool isBefore(TimeOfDay other) {
+    return hour < other.hour && minute < other.minute;
+  }
+
+  bool isAfter(TimeOfDay other) {
+    return hour > other.hour && minute > other.minute;
+  }
+
+  TimeOfDay add({int hours = 0, int minutes = 0}) {
+    return replacing(hour: hour + hours, minute: minute + minutes);
+  }
+}