In Ubuntu 17.10 with GNOME, how do I change the clock to a 12 hour format? And maybe for a more difficult rhetorical question, why does Ubuntu feel compelled to hide such basic functionality?
Click on Activities and search for "Settings" and launch it. Click on Details at the bottom of the sidebar and then select "Date & Time".
You'll find an option to change the time format.
In search bar, search for "Time" and the "Date & Time" setting comes up, click on it and change to "AM/PM".