How to change NTP server in airport express?
Although the GUI has lost a lot of functionality, there is a workaround.

In AU, select the device and click 'edit'. Under the File menu export the config, which you can then edit, and import it using the same menu. The relevant tag is

&lt;key&gt;ntSV&lt;/key&gt;
&lt;string&gt;time.apple.com&lt;/string&gt;


You will need to be very careful with your editing or you may render it unbootable! You will need to restart for the config change to take effect. I've done this with a Time Capsule: I expect that this is possible with Airport Express too, but do not have access to one to experiment with.