Save metadata as a timestamp, display as a “pretty” date

You don’t have to store it as a timestamp to order by date, yyyy/mm/dd will work just fine.

As for converting the format:

$date="2011/09/19";
$date = strtotime( $date );
echo date( 'F j, Y', $date );

see php’s strtotime and date

EDIT- oh, maybe I misunderstood, thought you said timestamp, not date. anyway, the conversion code in the second part is what you need. a unix timestamp is the number of seconds since January 1st, 1970.