I have the Airport Extreme

The only control in Airport Extreme is timed access. Is that what you mean?

Timed access is configured from airport utility.

See http://www.macworld.com/article/3017158/security/use-timed-access-control-to-res trict-when-devices-can-connect-to-your-apple-base-station-wi-fi.html

As one of 380,000 such posts.

You can usually find a youtube video.

https://www.youtube.com/watch?v=btfZYLiMb2M

And loads of posts by Bob Timmons around here.. complete with screenshots.

Airport Extreme Timed access

Here is a good link on the problem of midnight.

Can't set wireless timed access controls past 11:59 PM