Hi;

I allready do this, but not in PBP, however i will explain how i do it;
After receiving the GMT you convert it to EpochTime
Than you subtract the seconds to the EpochTime, for instance 8hourx3600 = 28800seconds
Then you convert the subtracted EpochTime to Normal Time again, that will be your new time with the 8 hours subtracted.

If you fo like Archangel said, you canno't only subtract the 8 hours, because we have to works out also the date, and not only time. Imagine its 2 am, if you subtract 8 hours it will be 6 pm, but the day will be diferent.