Find the count of a weekday between two dates without iterating/looping

public static class ExtendDateTime
{
    public int CountWeekDays(this DateTime thisdate, DateTime thatDate)
    {
        int days = Math.Abs((thisDate - thatDate).Days) + 1;
        return = ((days/7) * 5) + (days % 7);
    }
}

DateTime startDate = new DateTime(2011, 11, 1);
int weekdays = startDate.CountWeekDays(new DateTime(2011, 11, 30));