Add Date Last Modified next to Published Date in WordPress Posts.

daan-date-last-modified.php

<?php
/**
 * Plugin Name: Date Last Modified for WordPress Posts
 * Plugin URI:  https://daan.dev/wordpress/date-last-updated-modified/ 
 * Description: Display 'Last Updated' date along with 'Date Published' in WordPress.
 * Version: 1.0.0
 * Author: Daan van den Bergh
 * Author URI: https://daan.dev
 * License: GPL2v2 or later
 * Text Domain: daan-date-last-modified
 */

function daan_add_date_last_modified($date_published)
{
    global $post;

    $post_id = get_queried_object_id();

    // Make sure to only apply this filter on the current post, and not 'recent posts' widgets, archive pages, etc.
    if ($post_id != $post->ID) {
        return $date_published;
    }

    /**
     * get_post_datetime() return datetime-object which can be compared, formatted, modified, etc.
     */
    $modified_datetime  = get_post_datetime($post->ID, 'modified');
    $published_datetime = get_post_datetime($post->ID);

    /**
     * Exit if Date Last Modified is equal to or smaller than Date Published.
     */
    if ($modified_datetime <= $published_datetime) {
        return $date_published;
    }

    $text_before_date        = __('Updated', 'daan-date-last-modified');
    $before                  = '(' . $text_before_date . ': ';
    $after                   = ')';
    $modified_ymd            = $modified_datetime->format('Y-m-d');
    $modified_human_readable = $modified_datetime->format(get_option('date_format'));

    $updated_html = $before . sprintf(
            '<time class="entry-date" datetime="%s" itemprop="dateModified">%s</time>',
            esc_attr($modified_ymd),
            esc_attr($modified_human_readable)
        ) . $after;

    return $date_published . ' ' . $updated_html;
}

// ATTENTION: The first parameter in add_filter() is the filter's name. This might differ per theme.
add_filter('the_time', 'daan_add_date_last_modified', null, 1);