Once more with feeling.....
The timezones of your users are IRRELEVANT!!!! The timezone of your forum IS RELEVANT!!!!
The intent is SOLELY to display the forum's time/date stamps correctly when your forum observes "Daylight/Summer" time.
I think some find this issue complictaed due to the fact that there are two places to maintain timezones, one for the forum (board timezone), the other for the user (user profile timezone). The user timezone exists simply as a matter of convenience for the user. Your users can change their
timezones as they see fit. Unless you use the "Date for Humans" Mod, most users will never touch their timezones anyway. But they have no affect on the forum date/time stamps... they only effect how a user sees the date/times if
they Log in. If they do not log in, they will only
see the forum time/date stamps relative to the forum's timezone.
A second point of confusion may lie in the fact that only one time/date stamp is stored, but that time/date stamp can be displayed in a wide vairety of ways at the same time to different users with different profile timezone value settings. The display does not change the stored value... only the presentation.