One of the things I often need to do is to calculate the number of work days between 2 dates in order to calculate billable days. There is a very useful function called NETWORKDAYS which does exactly this. It works the same in both Excel and Google Spreadsheets
The syntax is =NETWORKDAYS (start_date, end_date, [holidays])
example =NETWORKDAYS (14/09/2009, 10/11/2009, 18/09/2009) gives you 41 days. In my example I only used one holiday but you can of course include multiple days in there.