Frontend first draft

frontend/pshared/lib/models/settings/time_validity.dart

@@ -0,0 +1,27 @@
+import 'package:json_annotation/json_annotation.dart';
+
+part 'time_validity.g.dart';
+
+
+@JsonSerializable()
+class TimeValidity {
+  final DateTime? start;
+  final DateTime? expiry;
+
+  const TimeValidity({this.start, this.expiry});
+
+  bool get isExpired => expiry?.isBefore(DateTime.now()) ?? false;
+  bool get isNotStarted => start?.isAfter(DateTime.now()) ?? false;
+  bool get isActive => (!isNotStarted) && (!isExpired);
+
+  TimeValidity copyWith({
+    DateTime? Function()? start, 
+    DateTime? Function()? expiry, 
+  }) => TimeValidity(
+    start: start == null ? this.start : start(),
+    expiry: expiry == null ? this.expiry : expiry(),
+  );
+
+  factory TimeValidity.fromJson(Map<String, dynamic> json) => _$TimeValidityFromJson(json);
+  Map<String, dynamic> toJson() => _$TimeValidityToJson(this);
+}