Allow translators to specify a default timezone string OR GMT offset. Stop...

Allow translators to specify a default timezone string OR GMT offset. Stop doing date('Z')/3600 math for default gmt_offset, as since WP 2.9, we set server time to UTC. Server time is unreliable, so no workarounds for now. fixes #19601.

git-svn-id: https://core.svn.wordpress.org/trunk@19785 1a063a9b-81f0-0310-95a4-ce76da25c4cd