When is Date() a string?

Simple example says it all:

<?php
include 'Date.php';
$date = new Date();
$d = $date->format('%Y-%m-%D');

if(is_string($date)){
echo "date is a string<br />";
}else echo "date is not a string<br />";

if(is_string($d)){
echo "d is a string<br />";
}else echo "d is not a string<br />";
?>

returns:

date is not a string
d is a string