if you use one of the email templates in Universal Email, it will automatically include code to write when the form is submitted:
<th style="font-size: 12px; width: 134px; text-align: right; padding: 3px 10px 3px 3px; font-weight: bold;">Form Submitted:</th>
<td style="font-size: 12px; padding: 3px;"><?php $now = time(); ?><?php echo date("n-j-Y", $now); ?> | <?php echo date("g:i A T", $now); ?></td>