Indicator Applet Complete only shows the time (in my UM 22.04 setup).
If your setup showed the full date and time format previously, without changing your indicator selection, then confirm your indicator's name by
- doing right-click on the indicator,
- noting the name given, then
- right-click on the panel to "Add Panel"
to verify that same indicator is still available by name.
ONLY if it is still available, then
- first remove the indicator from the panel,
- then re-Add the indicator App to the panel.
If you still don't get the "date and time" showing, then you need to use two separate Apps, like I do myself. Namely
- "Indicator Applet" at the extreme right, and
- "Clock" applet, just to the left of it.
That will give you what you want!
After you make your changes to your panel(s), be sure to save the new configuration as a "Named Layout" using MATE Tweak.
To do that, follow the guidance provided in the bottom half of the this posting.