Date to timestamp, er, datestamp?

<?php
$ymd_to_ts = function($ymd){
    list($y,$m,$d)=explode("-", $ymd);
    return 365 * $y + floor( $y / 4 ) - floor( $y / 100 ) + floor( $y / 400 )
        + 30 * $m + floor( ($m + ( $m > 8 )) / 2) + (($y % 4 == 0) - ($y % 100 == 0) + ($y % 400 == 0) ? 1 : 2) * ($m < 3)
        + $d - 719560;
    };