added frontend

hartmann-foto-documentation-frontend/lib/utils/date_time_utils.dart

@@ -0,0 +1,17 @@
+final class DateTimeUtils {
+  static DateTime? toDateTime(dynamic element) {
+    if (element == null) {
+      return null;
+    }
+    String text = element.toString();
+    int? time = int.tryParse(text);
+    if (time == null) {
+      return null;
+    }
+    return DateTime.fromMillisecondsSinceEpoch(time);
+  }
+
+  static int? fromDateTime(DateTime? dt) {
+    return dt?.millisecondsSinceEpoch;
+  }
+}