Fix: Passt an, dass bei monatlichen Transaktionen der Tag entsprechend hergenommen wird

lib/PlatformDependent/Web/date_utils_web.dart

@@ -16,4 +16,32 @@ class DateUtils {
     final DateTime monthDate,
     final int monthsToAdd,
   ) => DateTime(monthDate.year, monthDate.month + monthsToAdd);
+
+  /// Returns the number of days in a month, according to the proleptic
+  /// Gregorian calendar.
+  ///
+  /// This applies the leap year logic introduced by the Gregorian reforms of
+  /// 1582. It will not give valid results for dates prior to that time.
+  static int getDaysInMonth(final int year, final int month) {
+    if (month == DateTime.february) {
+      final bool isLeapYear =
+          (year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0);
+      return isLeapYear ? 29 : 28;
+    }
+    const List<int> daysInMonth = <int>[
+      31,
+      -1,
+      31,
+      30,
+      31,
+      30,
+      31,
+      31,
+      30,
+      31,
+      30,
+      31,
+    ];
+    return daysInMonth[month - 1];
+  }
 }