Cannot convert from Hijri Date to Gregorian date (c#)

here is the code it is working well now on this code I'm returning the date from the function as string not as datetime, but you can simply using return datetime type instead on string

public string ConvertDateCalendar(DateTime DateConv, string Calendar, string DateLangCulture)
{
    System.Globalization.DateTimeFormatInfo DTFormat;
    DateLangCulture = DateLangCulture.ToLower();
    /// We can't have the hijri date writen in English. We will get a runtime error - LAITH - 11/13/2005 1:01:45 PM -

    if (Calendar == "Hijri" && DateLangCulture.StartsWith("en-"))
    {
        DateLangCulture = "ar-sa";
    }

    /// Set the date time format to the given culture - LAITH - 11/13/2005 1:04:22 PM -
    DTFormat = new System.Globalization.CultureInfo(DateLangCulture, false).DateTimeFormat;

    /// Set the calendar property of the date time format to the given calendar - LAITH - 11/13/2005 1:04:52 PM -
    switch (Calendar)
    {
        case "Hijri":
            DTFormat.Calendar = new System.Globalization.HijriCalendar();
            break;

        case "Gregorian":
            DTFormat.Calendar = new System.Globalization.GregorianCalendar();
            break;

        default:
            return "";
    }

    /// We format the date structure to whatever we want - LAITH - 11/13/2005 1:05:39 PM -
    DTFormat.ShortDatePattern = "dd/MM/yyyy";
    return (DateConv.Date.ToString("f", DTFormat));
}

To call this Method here are an example

ltrCalValue.Text = ConvertDateCalendar(CalHijri.SelectedDate, "Gregorian", "en-US");

To call the Hijri

ltrCalValue.Text = ConvertDateCalendar(CalHijri.SelectedDate, "Hijri", "en-US");