Last updated date in WordPress

There are a few methods to display last updated in WordPress

Change post date to last updated date

But default, post date is published date, if like to change it to last updated date, add additional CSS as below.

/* for post modified */
.entry-date.published { display: none; }
.updated:not(.published) { display: inline-block; }

/* for post that never been modified, theme uses .updated */
.entry-date.published.updated { display: inline-block; }

Change functions.php

function my_last_updated_date( $content ) {
    $u_time = get_the_time('U');
    $u_modified_time = get_the_modified_time('U');

    if ($u_modified_time >= $u_time + 86400) {
        $updated_date = get_the_modified_time('F jS, Y');
        $updated_time = get_the_modified_time('h:i a');
        $custom_content .= '<p class="last-updated entry-meta">Last updated on '. $updated_date . ' at '. $updated_time .'</p>';
    }

    $custom_content .= $content;
    return $custom_content;
}
add_filter( 'the_content', 'my_last_updated_date' );

Change template-tags.php

This is the PHP way to modify the code to display dates.

References

How to replace published date?