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

$date = format_date($node->created,'large');

The second argument can be one of: small, medium, large, custom.

The third argument specifies a format for use with the "custom" option.

Here's a few handy PHP date format strings: