A couple of useful Drupal functions for formatting date and time. format_interval
$age = format_interval(time() - $node->created,1) . t(' ago');
The above outputs:
12 weeks ago
You second argument controls how many fields are outputted. If you passed "3" as the granularity option, then the output might look like:
12 weeks 4 days 16 hours ago
format_date
