How to set a timezone, using GMT / UTC offset?
// Set the default timezone using a GMT offset
$offset = -5; // GMT offset
$is_DST = FALSE; // observing daylight savings?
$timezone_name = timezone_name_from_abbr('', $offset * 3600, $is_DST); // e.g. "America/New_York"
date_default_timezone_set($timezone_name);