Расчет номера недели для определенной даты

public static int GetWeekNumber(DateTime dtPassed)
{
    CultureInfo ciCurr = CultureInfo.CurrentCulture;
    int weekNum = ciCurr.Calendar.GetWeekOfYear(dtPassed, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    return weekNum;
}

private void button1_Click(object sender, EventArgs e)
{
    DateTime date = new DateTime(2012,9,1);

    MessageBox.Show(GetWeekNumber(date).ToString());
}

using System.Globalization;