girovagando ho trovato anche questa:

function date_diff($dat1,$dat2) {

/* Dat1 and Dat2 passed as "YYYY-MM-DD" */

$tmp_dat1 = mktime(0,0,0,substr($dat1,5,2),substr($dat1,8,2),s ubstr($dat1,0,4));
$tmp_dat2 = mktime(0,0,0,substr($dat2,5,2),substr($dat2,8,2),s ubstr($dat2,0,4));

$yeardiff = date('Y',$tmp_dat1)-date('Y',$tmp_dat2);
/* a leap year in every 4 years and the days-difference */
$diff = date('z',$tmp_dat1)-date('z',$tmp_dat2) + floor($yeardiff /4)*1461;

/* remainder */
for ($yeardiff = $yeardiff % 4; $yeardiff>0; $yeardiff--) {
$diff += 365 + date('L',mktime(0,0,0,1,1,intval(substr((($tmp_dat 1>$tmp_dat2) ? $dat1 : $dat2),0,4))-$yeardiff+1));
}

return $diff;
}