<form name="whatever" action="script.php" method="POST">
<input type="hidden" name="usertime" value="" />
.....
<input type="submit" onClick="var d = new Date(); usertime.value = d.getHours()+':'+d.getMinutes()+':'+d.getSeconds()" />
</form>
this will give you their time in a 24 hour format, granted the person has javascript enabled.
access it through $_POST['usertime'] after the form is submitted. so check for an empty value to indicate they had js disabled.