Created basic Timer class and TimerEntity

Signed-off-by: Andreas Fahrecker <AndreasFahrecker@gmail.com>

lib/persistence/timer_entity.dart

@@ -0,0 +1,32 @@
+class TimerEntity {
+  final DateTime startTime;
+  final DateTime endTime;
+
+  TimerEntity(this.startTime, this.endTime);
+
+  @override
+  int get hashCode => startTime.hashCode ^ endTime.hashCode;
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is TimerEntity &&
+          runtimeType == other.runtimeType &&
+          startTime == other.startTime &&
+          endTime == other.endTime;
+
+  Map<String, Object> toJson() {
+    return {
+      "startTime": startTime.millisecondsSinceEpoch,
+      "endTime": startTime.millisecondsSinceEpoch
+    };
+  }
+
+  static TimerEntity fromJson(Map<String, Object> json) {
+    DateTime startTime =
+        DateTime.fromMillisecondsSinceEpoch(json["startTime"] as int);
+    DateTime endTime =
+        DateTime.fromMillisecondsSinceEpoch(json["endTime"] as int);
+    return TimerEntity(startTime, endTime);
+  }
+}