Changing date as "time ago" in template.php

I found the php code that controls the display of time on template.php:

if ($vars['submitted']) {
$vars['submitted'] = t('By !username | @date', array(
'!username' => theme('username', $node),
'@date' => format_date($node->created, 'custom', "F jS, Y g:i a"),
'!microdate' => format_date($node->created, 'custom', "Y-m-d\TH:i:sO")

is their a way to change this, so it shows how many days, hours, minutes ago?

thanks a lot i advance for any help