Is it required in the form for the user to actually enter the date in question? Or is the actual system date sufficient? You could create a hidden input in the form to include the desired date format for you. Perhaps the following is a simple alternative?
<?php
$mydate = date('Y-m-d');
?>
<form>
<input name="date" type="hidden" value="<?php echo $mydate;?>" />
</form>