The PT clock -should- be based off the server's clock that the website is hosted on. You can change the time offset in the admin's section for the forum but if the server is hosted in a country that is having DST that could be why

Also, some forum's scripts do automatically adjust for DST, not sure about vBulletin but I'm sure it does. There may even be an option in your settings, I don't recall but I know on a user level on some forum scripts you have the option for the time to automatically adjust for DST.