Here is an explanation.

Yes, but the differences are not integers. [...]

OK, there are a few exceptions. But I'm not sure whether we should change it as it complicates it a little bit with floats (text field instead of select box, different ways users insert the values...).

In my profile the time-difference is set to '0', the server sits in Germany (D-74321 Bietigheim-Bissingen), my clock shows 17:34 and the post shows up with the timestamp 16:34. Maybe the server slept over the daylight saving time.

Forum time = GMT.

Alex