The SCCA currently accepts two standards families for harnesses: SFI 16.1 and FIA (one of 8853/1895, 8853/98, or 8854/98). Belt expiration is determined differently for the two families.
For SFI belts, we take the date punched into the SFI label (which is approximately the date of sale in most cases, more on that in a minute), add 2 years, and then round out to the following December. For FIA belts, we just use the expiration date on the belt label. If a belt carries both certifications, we use the later date (giving longer usage). This invariably ends up being the FIA expiration date, which is commonly 5-6 years away for new FIA belts.
So what's the difference? It's pretty simple. Harness webbing is typically Nylon or Polyester; SFI standards permit either. FIA only permits Polyester, which has better aging characteristics, which mostly have to do with the response of the belt to UV exposure (FIA belts also only have cam-lock style latches, but this isn't really an aging related issue.)
So since FIA belts usually last nearly twice as long, I tend to advise drivers that the $30-$40 premium for FIA belts is well worth it.
As for the date on the SFI label, some belt vendors aren't careful. Sometimes this means that you open a box of belts and the date is still two months away. Sometimes it means that you open a box of belts and discover that they are 3 years old and have already expired. I recommend that when you receive a set of belts, check the date immediately. If you just put them in the car and show up, the Tech Inspector isn't likely to have mercy on you.