Add a new 'Last Modified' column to WordPress admin Post list pages

wordpress-add-new-column-for-last-modified-date.php

<?php

function add_new_admin_column($columns)
{
  $columns['last_modified'] = 'Last Modified';

  return $columns;
}

add_filter('manage_posts_columns', 'add_new_admin_column');
add_filter('manage_pages_columns', 'add_new_admin_column');

// get Last Modified date of post content and at this value to post_meta
function show_last_modified_column($name, $post_id)
{
  if($name === 'last_modified')
  {
    // Store the last modified date timestamp as a post meta option value
    $last_modified_timestamp = get_the_modified_time('U');
    update_post_meta($post_id, 'last_modified', $last_modified_timestamp);

    // Display the last modified date in a human readable format
    // echo the result to be displayed in the cell
    $date_format                  = 'Y/m/d';
    $time_format                  = 'g:i:s';
    $last_modified_human_readable = date($date_format . ' ' . $time_format . ' a', $last_modified_timestamp);

    echo 'Last Modified <br/>' . $last_modified_human_readable;
  }
}

add_action('manage_posts_custom_column', 'show_last_modified_column', 10, 2);
add_action('manage_pages_custom_column', 'show_last_modified_column', 10, 2);

function make_last_modified_column_sortable($columns)
{
  $columns['last_modified'] = 'last_modified';

  return $columns;
}

add_filter('manage_edit-post_sortable_columns', 'make_last_modified_column_sortable');
add_filter('manage_edit-page_sortable_columns', 'make_last_modified_column_sortable');

// Modify query and add 'last_modified' meta_key
function last_modified_column_orderby($query)
{
  if(!is_admin() || !$query->is_main_query())
  {
    return;
  }

  $orderby = $query->get('orderby');

  if('last_modified' === $orderby)
  {
    $query->set('meta_key', 'last_modified');
    $query->set('orderby', 'meta_value_num');
    $query->set('last_modified', true);
  }
}

add_action('pre_get_posts', 'last_modified_column_orderby');

// append 'last_modified' var to query so it can used to change order for this column
function last_modified_column_request($vars)
{
  if(isset($vars['last_modified']) && $vars['last_modified'])
  {
    $vars = array_merge($vars, [
      'meta_key' => 'last_modified',
      'orderby'  => 'meta_value_num',
    ]);
  }

  return $vars;
}

add_filter('request', 'last_modified_column_request');