Working out a tricky formula is one of the things I like about web programming so much. Date calculations in particular can often be pretty tricky. I've got a set of "events" (projects actually) that span any number of days. The start date and end date of each is stored in a database. I need to know how many weekdays occur during the time span of each. Here is how I worked it out in PHP:

1. Calculate the number of days between the two dates (including the start and end dates).
   
   $allDays = (strtotime($endDate) - strtotime($startDate)) / (60 * 60 * 24) + 1;
2. Determine the day of the week for both the start date and the end date (numerical - 1 through 7).
   
   $startDOW = date('w', strtotime($startDate)) + 1;
3. Calculate the number of weekdays found in full weeks
   
   $fullWeekdays = (floor($allDays / 7) * 5);
4. And the tricky bit - calculate the remaining weekdays
   
   $modulusWeekdays = $endDOW - $startDOW + 1 + ($startDOW == 1 ? -1 : 0) + ($endDOW == 7 ? -1 : 0);

if ($modulusWeekdays < 0) $modulusWeekdays += 5;
5. Add the number of weekdays in full weeks to the remainder of weekdays and there's the answer. The number of weekdays that occur between any two dates.

UPDATE:

Here's a tool I've created that you can use to find the number of weekdays or weekends in a given time period: http://gabegrayum.com/tools/datecalc/