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It replicates the functionality of Open Office's NETWORKDAYS function - you give it a start date, an end date, and an array of any holidays you want skipped, and it'll tell you the number of business days (inclusive of the start and end days! I've tested it pretty strenuously but date arithmetic is complicated and there's always the possibility I missed something, so please feel free to check my math.
The function could certainly be made much more powerful, to allow you to set different days to be ignored (e.g.
Rather than using mktime() and a loop, James can subtract the current timestamp from the timestamp of the date in question and divide that by the number of seconds in a day: Another usage could find itself in a class submitted by Kyle M Hall which aids in the creation of timestamps from the recent past for use with My SQL.