You should never assume register_global_variables is turned on. Even if it is, it's deprecated and you should never use it that way.
Refer directly to the $_POST or $_GET variables. Most likely your form is POSTing, so you'd want your code to look something along the lines of this:
<input type="hidden" name="date" id="hiddenField" value="<?php echo $_POST['date'] ?>" />
If this doesn't work for you right away, print out the $_POST or $_GET variable on the page that would have the hidden form field and determine exactly what you want and refer to it.
echo "<pre>";
print_r($_POST);
echo "</pre>";