One option that'll work with any version of PHP is to do what's already been suggested, which is something like this:
$eventTime = '2010-04-28 17:25:43';
$age = time() - strtotime($eventTime);
That will give you the age in seconds. From there, you can display it however you wish.
One problem with this approach, however, is that it won't take into account time shifts causes by DST. If that's not a concern, then go for it. Otherwise, you'll probably want to use the diff() method in the DateTime class. Unfortunately, this is only an option if you're on at least PHP 5.3.
