Seconds. With the caveat that you can't use 0, so Sun Time is 24h worth of seconds.Can I get some insight to the values and how they're figured out? What if I wanted Sun Time +8h?
As for how I found it: I edited a save file with PKHex, figured which value it was writing for the time offset, and started searching the memory for it. Easier done with an unusual time offset (I used Sun+7h, only three matches for that value in the 70+ MB used by the game, two of which could be discarded out of hand). So, credits to the PKHex team for figuring how the time offset works, without them I would never have created this code.