How to show the age of each post in an ACF relationship field. For more info see https://philkurth.com.au/tips/customise-the-post-titles-in-acf-relationship-field/

filter-acf-relationship-field-post-titles.php

<?php

add_filter( 'acf/fields/relationship/result/name=related_posts', function ( $title, WP_Post $post, $field_arr ) {

	$posted_at = get_post_time( 'U', false, $post->ID );
	$now = current_time( 'timestamp' );
	$diff = human_time_diff( $posted_at, $now );

	return $title . sprintf( ' (%s ago)', $diff );
}, 10, 3 );