Find the number of working days between two dates inclusively
Personally, I think your approach is very complicated for what it should be trying to do: loop over all days between the start and the end date and count how many of those are not in the weekend.

Translating that sentence to code, gives me this:

private static int GetNumberOfWorkingDaysJeroen(DateTime start, DateTime stop)
{
    int days = 0;
    while(start &lt;= stop)
    {
        if(start.DayOfWeek != DayOfWeek.Saturday &amp;&amp; start.DayOfWeek != DayOfWeek.Sunday)
        {
            ++days;
        }
        start = start.AddDays(1);
    }
    return days;
}


I find this much easier to interpret than a bunch of modulos, casts and divisions.