help with dates

[rustybear at gmail.com]

[sorry about the earlier post. accidently hit enter]

I'm working on a website for some firefighters that want to be able to
sign-up for overtime and I need some help figuring out a date related
problem.

Here's the scenario:

Four groups of firefighters (group1, group2, group3, group4). Each
group works a 24 hr shift. So group1 works April 1, group2 works April
2, group3 works April 3, group4 works April 4, group 1 works April 5,
etc. It just keeps rolling like this forever into next year, etc.

I need to come up with a methodology for the following:

If given a date, find out what group is working on that date.

Has anyone dealt with a problem like this before?  Obviously I'll need
to have a starting date where group1 is associated with a starting
date. Fortunately there are no surprises. Groups work in order
(1,2,3,4,1,2,3,4,1,2,...) and dates go in order and there are
absolutely no special exceptions for holidays and anything else.

Thanks in advance for any clues.

Kevin