drupal 7, add date field with popup calendar to custom form.

scratch.txt

$form['startdate'] = array(
    '#type' => 'date_popup',
    '#title' => t('Lesson start date'),
    '#timepicker' => 'timepicker',
    '#timepicker_options' => array(
     'rows' => 6,
     'minutes' => array(
       'starts' => 0,
       'ends' => 56,
       'interval' => 4,
     ),
     'showCloseButton' => TRUE,
     'closeButtonText' => t('Close'),
    ),
    '#required' => TRUE,
    '#date_label_position' => 'above',
    '#default_value' => isset($lesson->startdate) ? date('Y-m-d H:i', $lesson->startdate) : '',
);