You could try turning the two dates into timestamps and then calculating the difference.
<?php
$date1="2001-12-19";
$date2="2001-12-23";
$bits=explode("-", $date1);
$tstamp1=mktime(0, 0, 0, $bits[1], $bits[2], $bits[0]);
$bits2=explode("-", $date2);
$tstamp2=mktime(0, 0, 0, $bits2[1], $bits2[2], $bits2[0]);
$diff=$tstamp2-$tstamp1;
$dayspassed=(($diff60)24);
echo "Days passed ".$dayspassed;
?>
I think that will work.
Mark