The key is the year of manufacture. Belts made in Jan '03 expire the same as belts made in Dec '03. It just that you get a full years (2003 season) additional use out of the set made in Jan '03.
BTW - just about all of the belt makers predate their belts. So if you bought a set today i.e. Dec '04, they are probably sitting on the shelf with Jan '05 dates on the label. And you can then run them in '05, '06, thru Dec '07. The belt companies do this so as to not 'waste' a year near the end of the season.
Or you can spend a little more on FIA belts and they are good for 5 years as the previous poster noted, but make sure they are ordered with this label present!
I got kinda ticked about the belt rule changes because I, like most people, read it to mean I have to replace the belts every two years. Since then I realized that the rule effectively means every three seasons and not every two years, and I found that much easier to swallow. Works out to about $25-$30 per season for new belts when you spread out the costs. Keep that in mind as well when pricing FIA belts, since they can be slightly more expensive but you spread that cost over 5 seasons.
[This message has been edited by Greg Gauper (edited December 21, 2004).]