GMT+13 is our time with daylight savings enabled, so technically GMT+12 yes. So perhaps if we just ignore and remove DST from the board and users will the issue go away? I'd rather have the time 1 hour out than have this weird thing going on.
Master time for the board in AdminCP:
http://i39.tinypic.com/a4bzfp.png (link as i haven't resized it yet)
Individual member settings:
Note that individual settings for DST doesn't seem to matter when determining who will be affected; however I am setting DST to that in user accounts and making them visit the userCP to get it to take effect. No change. Still random.