AM/PM in Login Screen Clock (18.04/20.04)

Ubuntu 18.04 / 20.04:

This can change "13:20" to "1:20 PM".

Edit this file:

/etc/gdm3/greeter.dconf-defaults

Make sure that this line is un-commented:

[org/gnome/desktop/interface]

Add this beneath it:

clock-format='12h'

Save and restart your system.