SharePoint: How to calculate week number using a formula

If you ever wish to use a date (say, modified date or creation date, for example) and have SharePoint determine which week number it occurs on, Microsoft has kindly done the work for you:

=INT(([Start Date]-DATE(YEAR([Start Date]),1,1)+(TEXT(WEEKDAY(DATE(YEAR([Start Date]),1,1)),"d")))/7)+1

More explanation, if desired, can be found at this blog.