I just installed Ubuntu MATE 20.04 and realised that some of the help topics in the community reference the older
dconf command for changing the time to 12 hour format.
In 20.04 using the Terminal :
gsettings set com.canonical.indicator.datetime time-format '12-hour'
- If your locale uses 12 hour, this should be set automatically.
- Some locales use both, so this command is if you prefer AM/PM.
It would be nice to have this as a graphical option, however, Ubuntu MATE uses Canonical's date/time indicator as opposed to the MATE Clock applet. This keeps it consistent with other indicators (and the feel of pre-Unity Ubuntu)