How would we limit the drop down for the Date's Timzone field? We are using the Date module that allows for enabling of Timezone during node edit (node/edit). We noticed that the dropdown is really long and we were wondering if there was a way to shorten the list to just say ETC, PST, UTC, GMT, etc. 

We tried all the configuration setting, but could not shorten the drop down. 

Current scenario:



Desired scenario:

Timezone (as an example)
 - UTC
 - PST/PDT
 - MST/MDT
 - EST/EDT
 - GMT
The Date module's field elements are supplied by the Date API module. The timezone component is supplied by its process callback date_timezone_element_process(). 

It triggers hook_date_timezone_process_alter() which can be evoked by a custom module to alter the options within $element['timezone']['#options'].

For example:

/**
 * Implements hook_date_timezone_process_alter().
 */
function MYMODULE_date_timezone_process_alter(&amp;$element, &amp;$form_state, &amp;$context) {

  if (isset($element['timezone']['#options'])) {
    // @TODO: Supply an array filter function that filters
    // IANA TZ names that don't correspond to
    // UTC - PST/PDT - MST/MDT - EST/EDT - GMT

    // Example: Filter TZ's that are only in the Americas
    $element['timezone']['#options'] = array_filter($element['timezone']['#options'], function($key) {
      return strpos($key, 'America/') !== FALSE;
    }, ARRAY_FILTER_USE_KEY);

  }
}