How can I change the year range displayed by a FAPI date field?
If you don't want to change every date field—which might have side effects else where on the site—you can easily do it on specific form elements.

function example_form(&amp;$form, &amp;$form_state)  {
  $form['birthday'] = array(
    '#type' =&gt; 'date',
    '#title' =&gt; t('Birthday'),
    '#required' =&gt; TRUE,
    // Add a second process function so we can change the years:
    '#process' =&gt; array('form_process_date', 'example_fix_year_range'),
  );

  return $form;
}

/**
 * Follow form_process_date() and re-do the years.
 */
function example_fix_year_range($element) {
  $year = format_date(REQUEST_TIME, 'custom', 'Y');
  $element['year']['#options'] = drupal_map_assoc(range($year - 120, $year));
  return $element;
}