Node Publish date form fields updated to use HTML5 elements. New html5 input types for date and time have been added to core. Switching to browser-supported date and time fields will remove our dependance on the core date module, and reliance on the jQuery UI datepicker.

before:

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

after:

$form['author']['date'] = array(
  '#type' => 'html_datetime',
  '#title' => t('Authored on'),
  '#default_value' => array(
    'date' => format_date($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.'),
);
Introduced in branch: 
1.15.x
Introduced in version: 
1.15.0
Impacts: 
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/3972