Manual:$wgLocalTZoffset/ja

詳細
Set an offset from UTC in minutes to use for the default timezone setting for anonymous users and new user accounts. Also used for users who set the corresponding text field in their preferences to an empty string. この設定は、ソフトウェアにおけるほとんどの日付/時間の表示に使用され、利用者の個人設定でオーバーライドできます. 署名のタイムスタンプには使用されません.

Leave at NULL to show times in universal time (UTC/GMT), or specify an offset in minutes.

例
To avoid problems with daylight saving time, you can have PHP calculate the offset for you. date('Z') は、タイムゾーンのオフセット (秒単位) を返します. The offset for timezones west of UTC is always negative, and for those east of UTC is always positive: