How to change the label of the "To:" field in the Date Module
If you look further on down in date_elements.inc at line 412 you'll notice:

drupal_alter('date_combo_process', $element, $form_state, $context);


which allows any module to alter what it's created, eg, something like this:

function YOURMODULE_date_combo_process_alter(&amp;$element, &amp;$form_state, &amp;$context) {

  $element['value2']['#title'] = t('expires:'); // this will change that to: to expires:

}


Just realize this simple example will do this for every instance of a combination to/from date, but information to get more specific is contained in $element, $form_state and $context.  For example, the specific field name is contained in $element['#field_name'] and the form_id is in $form_state['build_info']['form_id'].