Managing who gets alerts - shifts and rotations

We're using email, plus a script that rotates the weekly oncall alias
files. But we don't have a similar setup for the shift changes, and I
don't want to be manually managing 7x24 alias files. We can start
writing a tool to do that but I was really hoping that such a tool
already existed! Seems like we can't possibly be the first company to
need this.

(Nagios has more advanced capabilities in this regard but I would
rather be using xymon...since we're already all set up with BB)