Before converting times and date, you should set the correct timezone in PHP so the engine knows the time characteristics of the zone you are using:
date_default_timezone_set('America/Mexico_City');
$start_date = date('c', time()); // ISO date 8601 of "right now"
$start_zone = date('O', time()); // TZOFFSETFROM format of "right now"
date_default_timezone_set('America/Denver');
$to_zone = date('O', time()); // TZOFFSETTO of "right now"
Hope this may help