How I solved it is: (bear in mind that I am lazy and do not code properly) I just used php to translate the raw date string to a unix timestamp, do the math, then (if necessary) translate back to the date string on the fly.
It's for a little auction app that the guy wanted to have the auction close either at the close time listed, or 15 minutes after the last bid - thus the time reset thing. I wanted the date stored in the mysql format because the guy who is going to admin the thing is not that savvy, and he has to put the date in a form manually for each auction he does. So it works great. It's a fairly light traffic, big ticket item auction. My solution might not do well for heavy traffic.