At first, I thought that the timezone the servers were located in may be UTC, but from other MetaSO questions I see that the servers are located in NYC.
This question seems to indicate that the clock on the servers is set to UTC.
Is this the reason why reputation seems to have a display error? See the image below. "Today" I received some
+2 reps, for mods done "13 hours ago"...... but 13hr ago was 9/10/2013 for me, not 9/11/2013.
note; all these little reps are tag-cleanup that I'm doing.