Ahh, you can set the timezone on your PHP, but not on WXForum.net's PHP .. that is set to America/New_York and the SMF forum only allows a +/- hour offset (not a timezone name) to be used for displays. I'm fairly certain the data is actually stored with a UTC timestamp, and the display routines add +/- hours to the date() display.
It would be sweeter if the forum allowed you to pick your timezone from a list, then it would always be in sync, but sadly, that's not the way SMF was written.