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));

I've been paid as a programmer since 1982 with experience in Pascal, and C++ (both self-taught), and began writing Windows programs in 1991 using Visual C++ and MFC. In the 2nd half of 2007, I started writing C# Windows Forms and ASP.Net applications, and have since done WPF, Silverlight, WCF, web services, and Windows services.
 
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