Say host server is located in UK on GMT, serving country with +12hr time zone as set within vBulletin and Photopost.
Unregistered users show correct times in forum and in photopost - no problem
Now login with user account based in +12 timezone. The
vB forum displays correct time zone, but photopost displays time +12 hours, so a photo upload at 6pm now displays as as being uploaded at 6am the next day.