Default guest timezone 'by country'

[woostar]

I currently have my forum set to US Central Time but want to change this for UK guests.
I have an IP lookup that puts the guests country code into $countryCode.

What hook and variable would I have to put in the following code to override the default time format for UK users?

PHP Code:

// what hook?

if ($countryCode == 'GB' && !$show['member']) {

    // change offset
    $XXXXXXX == 0; // what variable controls time zone offset?

} 


Any help would be great,
Ta.