New date and time form elements: html_date, html_time, and html_datetime

$form['author']['date'] = array(    
  '#type' => module_exists('date') ? 'date_popup' : 'textfield',      
  '#title' => t('Authored on'),     
  // Note #date_format only applies if date module is enabled.      
  '#date_format' => 'Y-m-d g:i a',        
  '#default_value' => !empty($node->date) ? $node->date : NULL,
  '#description' => t('Leave blank to use the current time.'),
);

$form['author']['date'] = array(
  '#type' => 'html_datetime',
  '#title' => t('Authored on'),
  '#default_value' => array(
    'date' => format_date($node->created, 'custom', DATE_FORMAT_DATE),
    'time' => format_date($node->created, 'custom', DATE_FORMAT_TIME),
  ),
  '#attributes' => array(
    'date' => array(
      'min' => '1970-01-01',
      'max' => '2037-12-31',
      'placeholder' => t('e.g. @date', array(
        '@date' => format_date(REQUEST_TIME, 'custom', DATE_FORMAT_DATE)
      )),
    ),
    'time' => array(
      'placeholder' => t('e.g. @date', array(
        '@date' => format_date(REQUEST_TIME, 'custom', DATE_FORMAT_TIME)
      )),
    ),
  ),
  '#description' => t('Leave blank to use the current time.'),
);