How do I get the timezone form reference on a date field?
Use hook_date_timezone_process_alter in your custom module to do the magic, see below example and adapat if for you needs

function MODULENAME_date_timezone_process_alter(&amp;$element, &amp;$form_state, $context) {
  // add some logic here
  // keep only the needed values
  // dsm($element['timezone']['#options']);
  $element['timezone']['#options'] = array(
    'Africa/Abidjan ' =&gt; 'Africa/Abidjan',
    'America/Detroit' =&gt; 'America/Detroit',
  );
}