How to add a date popup calendar onto a custom form

How to use a date popup calendar within your custom module.

First, I need to download the Date module, and make my module dependent on date_popup by adding the following line into my module’s .info file.

dependencies[] = date_popup

Within my form builder function:

$form['date'] = array(
  '#title' => t('Arrival date'),

  // Provided by the date_popup module
  '#type' => 'date_popup',

  // Uses the PHP date() format -
  '#date_format' => 'j F Y',

  // Limits the year range to the next two upcoming years
  '#date_year_range' => '0:+2',

  // Default value must be in 'Y-m-d' format.
  '#default_value' => date('Y-m-d', time()),