Frontend first draft

frontend/pshared/lib/utils/datetime_serializer.dart

@@ -0,0 +1,9 @@
+extension DateTimeSerializer on DateTime {
+  static String toBackendString(DateTime dt) {
+      return dt.toUtc().toIso8601String();
+    }
+
+  static DateTime fromBackendString(String dateStr) {
+    return DateTime.parse(dateStr); 
+  }
+}