{tech}.Ben Buckman.net

I spent a long time today trying to figure out how to customize a Date field in Drupal 6. The field is called field_recurring_dates and looked like this:

I wanted the "To" date to only show the time, no date - so the field has Date, Start Time, and End Time. The field is repeating/"unlimited" so the whole unit gets returned via AHAH with the "Add Another Item" button. Renaming that to "Add another day" was easy, but getting the date/time fields to change was elusive. I tried everything else I could think of: #process, theme overrides for every element, preprocess, with no luck. The field is generated by a hook_elements inside the Date module, and I couldn't find the equivalent of a hook_elements_alter ... until I asked on IRC and got the perfect solution from Fox: the Form API's #after_build callback.

So here's the code, finally working, inside a custom module:

function MODULE_form_alter(&$form, $form_state, $form_id) {
  // work on initial node form and AHAH-generated partial form
  if ($form['#id'] == 'node-form' || $form_id == 'content_add_more_js') {
    if (isset($form['field_recurring_dates'])) {
      $form['field_recurring_dates']['#after_build'][] = 'custom_date_combo_after_build';
    }
 
    // rename the 'Add' button
    if (isset($form['field_recurring_dates']['field_recurring_dates_add_more'])) {
      $form['field_recurring_dates']['field_recurring_dates_add_more']['#value'] = 'Add Another Day';
    }
 
  }
}
 
function custom_date_combo_after_build(&$element) {
  // can be MULTIPLE items
  // need to find all the NUMERICALLY-KEYED elements
  foreach($element as $key => &$field) {
    if (is_numeric($key) && is_array($field) && isset($field['value']) && isset($field['value2'])) {
      // remove the Date part of 'To Date's
      unset($field['value2']['date']);
 
      // rename 'From Date' => 'Date'
      $field['value']['#title'] = 'Date';
 
      $field['value']['time']['#title'] = 'From Time';
      $field['value2']['time']['#title'] = 'To Time';
 
      // remove 'To date' block title
      $field['value2']['#title'] = '';
 
      // only need helper text outside whole group
      unset($field['#fieldset_description']);   
    }
  }
  return $element;
}

With a little CSS, it now looks like this:

Perfect!

Update and Correction: The above line,
unset($field['value2']['date']);
didn't work, because the missing 2nd date field messed up the field's validation.

So I checked out the Date Single Day module mentioned in the comments, and it's pretty much identical to this method except for this change:
$field['value2']['date']['#type'] = 'hidden';   // not unset()!

That seems to work well.
As a side note, I tried the #date_format attribute mentioned in the comments and couldn't get that to work at all. Also the first and second dates aren't separated in the form array enough to set a different #date_format on each. So #after_build seems to be the best bet.