unlang time / date comparison

Hi, Hopefully a simple question. I've looked around for a while but can't find the answer to this. I'd like to be able to take a date/time from a sql database, use unlang (not a module) to compare it to the current time and make a decision based on the fact that it's, for example, less or more than 3600 seconds/60 minutes/1 hour etc ago. I can't really work out the comparison of dates, or simple mathematical functions for that matter. Perhaps I'm missing something obvious - apologies if so, I've not been using FR for that long. Thanks as always. Andy