Fix: Macht die BackgroundWorker für den IsolateManager on Web möglich

lib/PlatformDependent/Web/date_utils_web.dart

@@ -0,0 +1,19 @@
+/// Utility functions for working with dates.
+class DateUtils {
+  /// Returns a [DateTime] that is [monthDate] with the added number
+  /// of months and the day set to 1 and time set to midnight.
+  ///
+  /// For example:
+  ///
+  /// ```dart
+  /// DateTime date = DateTime(2019, 1, 15);
+  /// DateTime futureDate = DateUtils.addMonthsToMonthDate(date, 3);
+  /// ```
+  ///
+  /// `date` would be January 15, 2019.
+  /// `futureDate` would be April 1, 2019 since it adds 3 months.
+  static DateTime addMonthsToMonthDate(
+    final DateTime monthDate,
+    final int monthsToAdd,
+  ) => DateTime(monthDate.year, monthDate.month + monthsToAdd);
+}