You've all come up with fascinating questions and answers. Thanks a lot.

I should clarify that the system was never required to do leave, resolve conflicts, remember staff preferences etc, just work as a kind of content-management system for one page, the "who's working when?" page. As Abigail said, just a dumb front end to a database.

Questions like this are becoming more interesting to me now, I note, whereas before I would have just started writing something right away, I'm now much more concerned with the structure before I start on the code.