dont know if this is the right place for it but the file
/_ep_phone_modules/endpoint/grandstream/gxp/$mac.cfg
has the line
P64 = {$gmtoff}
in it which is in seconds
it should be
P64 = {$timezone}
which is in minutes
I tried to file a bug but i lack permissions
It looks like a similar thing happened with the handytone section, so it might be worth checking the other grandstreams.
This is in freepbx 2.8.1.4
