1900 Date System converter to PHP takes $date as unix time and returns days since 1900.

get_1900_date.php

<?php
function get_1900_date($date) {
    $start = strtotime("1900-01-01");
    $diff = $date - $start;
    $days = $diff / 86400;
    return round($days, 0);
}
?>