Sorry if its confusing lets try and make it clearer.
Every forum application except vb
uses whats known as a timeoffset which adjusts the time from server time which means users can individually select an offset to show things in there own time
does it a bit different. They adjust before the get go all server time for posts to GMT and then the users offset is set to there timezone
Do not worry about your timezone as thats already set in your user offset. You need to add back in or subtract the time where ever your server is at to GMT. You do not put a + for positive numbers. I posted an example so you would not be confused.
My server is in mountain time US so I need to put 7 in the GMTOFFSET box in photopost. So your in California but that makes no nevermind as you need to know where your server is at. Most likely your going to be putting in a 7 or a 6 in that GMT offset box.