c# Номер недели в месяце

private int GetWeekOfMonth (DateTime date)
{
    DateTime beginningOfMonth = new DateTime(date.Year, date.Month, 1);

    while (date.Date.AddDays(1).DayOfWeek != System.Globalization.CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek)
        date = date.AddDays(1);

    return (int)Math.Truncate((double)date.Subtract(beginningOfMonth).TotalDays / 7f) + 1;
}