function wcfm_custom_add_invoice_date( $document_type, $order ) {
    ?>
    <tr class="invoice-date">
        <th><?php _e( 'Invoice Date:', 'wc-frontend-manager-ultimate' ); ?></th>
        <td><?php echo date_i18n( wc_date_format(), current_time( 'timestamp', 0 ) ); ?> @<?php echo date_i18n( wc_time_format(), current_time( 'timestamp', 0 ) ); ?></td>
    </tr>
    <?php
}

add_action( 'wcfm_pdf_invoice_after_order_data', 'wcfm_custom_add_invoice_date', 999, 2 );