RLB,
I did this to test this:
Header
{nwa_cookiecheck}
{assign var=Hi value='Hi'|date}
{if $Hi < "1600"}
<h3>Service Unavailable</h3>
<h4>Guest Network Access unavailable before 4pm</h4>
<div style="display:none;">
{elseif $Hi >= "1700"}
<h3>Service Unavailable</h3>
<h4>Guest Network Access unavailable after 5pm</h4>
<div style="display:none;">
{/if}
{if $errmsg}{nwa_icontext type=error}{$errmsg|escape}{/nwa_icontext}{/if}
{nwa_text id=7980}<p>
Please login to the network using your
username and password.
</p>{/nwa_text}
Footer
{if $Hi < "1600" || $Hi >= "1700"}
</div>
{/if}
{nwa_text id=7979}<p>
Contact a staff member if you are experiencing
difficulty logging in.
</p>{/nwa_text}
I purposely changed to between 1600 and 1700 so that I would be outside of that range to test. You can change it to any 24-hour range to suit your needs.