Well, if you are checking this after submitting a form, the following might work:
<form >
<!-- form stuff -->
<script language="JavaScript">
<!--
today = new Date ();
document.write ('<input type="hidden" name="client_date" value="' + today + '" />');
//-->
</script>
</form>
I'm a shitty JavaScript programmer and this code most probably doesn't work 🙂 But I hope you get the concept; Posting javascript varables via form and grabbing them with PHP on the 'other side'
regards,
thom