Go to admin - settings - regional. There is a time difference setting there.

:D

my mean is time difference
for example , i have +3.5 diffrence time from GMT . how can I change it?

Currently you can't do partial hours. You can do +3 or +4. It's an issue that's been raised and MIGHT be fixed in upcoming issues. For now, unfortunately, you can only offset your blog from your server by full hours.

Oh by the way: you don't set your time difference compared to GMT. You set it up to be the difference from your time to your server's time.