... Click Network, select Enable Access Control, and then click Timed Access Control.
Click Add (+) and enter the MAC address and description or name of the computers you’re letting access the network...
- Airport 5.6: wifi -Use AirPort 5.6 Advanced:Logging & Statistics:Logs & Statistics:DHCP Clients from the DHCP serving device (only device unless you have bridged network like mine). You can copy paste the MAC address.
- AirPort Utility 6.x: Find IP, Mac, and network name for WiFi (not ethernet) devices by holding option key, then double click on the AirPort advice providing DHCP services. You will see a new 'Summary' tab, click outline icon in wireless client list to see details. The MAC address is called a "Hardware Address". You can't copy paste it (insert sound of teeth grinding).
- AirPort Utility for iOS: (Best). Tap each device on your network that can have wifi clients. Tap "Wirless Clients" then tap name. See IP, Hardware address connection details. You can tap Hardware Address to copy it. Mail or otherwise share it to your Mac so you can set timed access; you can't configure Timed Access from an iOS device.
From the Mac address you can set timed access control.
- Airport Utility 6.0 - Mac OS X Hints: how to install Airport Utility 5.6 using Pacifist (there are some free utilities for this too, google it)
- Was AirPort Utility 6 the start of Apple's year of drifting dangerously? 6/2013. Mavericks gives me hope Cook might turn this ship around.
- fn -
 For an iOS device the Name might appear in iTunes, but I think spaces get replaced with hyphens. Better to use the MAC address.