Manual:$wgLocalTZoffset

Details
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. This setting is used for most date/time displays in the software, and is overrideable in user preferences. It is NOT used for signature timestamps.

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

Examples
To avoid problems with daylight saving time, you can have PHP calculate the offset for you. date('Z') returns the timezone offset in seconds. The offset for timezones west of UTC is always negative, and for those east of UTC is always positive: