/**
 * Form builder.
 */
function mymodule_myform($form) {
  $form['date'] = array(
    '#type' => 'textfield',
    '#title' => t('Date'),
    '#size' => 9,
    '#attributes' => array(
      'class' => array('datepicker'),  
    ),
  );
 
  $form['#attached']['library'][] = array('system', 'ui.datepicker');
  $form['#attached']['js'][] = drupal_get_path('module', 'mymodule') . '/mymodule.js';
 
  return $form;
}

(function ($) {
  Drupal.behaviors.mymodule = {
    attach: function (context, settings) {
      $('.datepicker', context).datepicker();
    }
  };
})(jQuery);

Drupal 7