you could try dayofweek function, with it help you could exclude (sat,sun) from your results  (Sun = 1, Sat = 7)

What if a client spent 60 days on a project?
It wouldn't be 10 days worth of weekends.

Also, here is a trickier case:

Suppose I started working on wednesday 8th and finished on monday 27th.

Thus, without deducting any weekends, I have apparently spent 20 days on the project.

2 weekends have definitely passed (so Wed 22nd would be the start of my third week) but then in the remaining 6 days, I happened to run into another weekend.

This would probably need to be considered as well.

I also thought about this problem for some time at one point: http://www.crystalreportsbook.com/Forum/forum_posts.asp?TID=13368&KW=

Edited by Keikoku - 23 Jun 2011 at 9:51am

Basically, the total number of weeks that have passed and the remaining days should be calculated separately.

But it will only be off by a maximum of 2 days so if you do not need such accuracy (ie: auditing) then it doesn't matter.