Custom form with Date textfield with jQuery popup
You need to initialize the form with drupal_get_form():

$form = drupal_get_form('MYMODULE_birthdate_form');
$html = drupal_render($form);

function MYMODULE_birthdate_form($form, &amp;$form_state) {
  $form['birthdate'] = array(
    '#type' =&gt; 'date_popup',
    // ...
  );
  return $form
}