codice:
select to_days('2006-03-09') - to_days('2006-02-10') as diff  from ...etc.
se invece hai mysql >= 4.1 ...
codice:
SELECT DATEDIFF('2006-03-09', '2006-02-10') AS diff from ... etc.
se invece fai tutto su php usa list, explode, mktime e ricavato unix timestamp dividi per (60 * 60 * 24)