dateWidget.php

<?php
 /**
   * Implements WidgetInterface::getPreviewForm().
   *
   * Return a form to collect the date information from the user.
   */
  public function getPreviewForm($element, &$form_state) {
    $element['#type'] = 'fieldset';
    $element['#title'] = empty($this->settings['title']) ? t('Date/Time:') : $this->settings['title'];
    $element['#description']= t('Preview nodes published on or after this date.');
    $element['#attributes']['class'] = array('sps-date-widget');

    $element['preview_date'] = array(
      '#type' => 'date_popup',
      '#title' => t('Date to Preview'),
      '#default_value' => isset($form_state['values']['preview_date']) ? $form_state['values']['preview_date'] : NULL,
    );

    //have a fallback if they don't have date_popup installed.
    if (!module_exists('date_popup')) {
      $element['preview_data']['#type'] = 'date';
      $element['preview_time'] = array(
        '#type' => 'textfield',
        '#title' => t('Time'),
        '#size' => 9,
        '#default_value' => isset($form_state['values']['preview_time']) ? $form_state['values']['preview_time'] : '00:00:00',
      );
    }

    return $element;
  }