public class StringToDateTimeConverter : IValueConverter
    {

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value == null)
            {
                return null;
            }
            return ((DateTime)value).ToString(parameter as string, new CultureInfo("de-DE"));
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (string.IsNullOrEmpty(value as string))
            {
                return null;
            }
            try
            {
                var theValue = (value as string);

                if(theValue.Length == 4)
                {
                    string[] subString = new string[2];
                    subString[0] = theValue.Substring(0,2);
                    subString[1] = theValue.Substring(2);
                    theValue = subString[0] + ":" + subString[1];
                }
                else if (theValue.Length == 8)
                {
                    string[] subString = new string[3];
                    subString[0] = theValue.Substring(0, 2);
                    subString[1] = theValue.Substring(2, 2);
                    subString[2] = theValue.Substring(4);
                    theValue = subString[2] + "-" + subString[1] + "-" + subString[0];
                }

                DateTime dt = DateTime.Parse(theValue as string, new CultureInfo("de-DE"));
                return dt as DateTime?;
            }
            catch (Exception)
            {
                return null;
            }
        }
    }