Future and past $core->user_server_tz_diff dst failure.

[TitanKing]

Because the $core->user_server_tz_diff function used current timestamp dates to determine timezone differences, dates in the future and past calculations on date formats was incorrect, either by incrementing or decrementing the date value incorrectly because of changing DST.

This was solved by allowing to pass a future or past timestamp so that timezone differences can be calculated on the correct DST.