Added api for custom timetable events

lib/api/mhsl/mhslApi.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
+import 'package:jiffy/jiffy.dart';
 import '../apiError.dart';
 import '../apiRequest.dart';
 
@@ -27,4 +28,8 @@ abstract class MhslApi<T> extends ApiRequest {
 
     return assemble(utf8.decode(data.bodyBytes));
   }
+
+  static String dateTimeToJson(DateTime time) => Jiffy.parseFromDateTime(time).format(pattern: "yyyy-MM-dd HH:mm:ss");
+  static DateTime dateTimeFromJson(String time) => DateTime.parse(time);
+
 }