bbPress.org

You were very close !

function change_reply_post_date($output) {
$day= '('.get_post_time('D').')';
$output = preg_replace( '/at/' , $day , $output );
return $output;
}
add_filter( 'bbp_get_reply_post_date' , 'change_reply_post_date');

the line $day…etc. sets $day equal to an ‘open bracket’, followed by the day, followed by a ‘close bracket’ – the full stops are a concatenate in php.

your original "<?php echo get_post_time('D'); ?>" does the following
<?php = switch on php – we’re already in php, so not needed
echo = send to screen – but we don’t want that, we want the result to be sent, which will be done by other code
get_post_time= that’s the key part that you had figured
?> = ends php, which we don’t want to do as the rest of the line is php

great, glad you are fixed !!