My date-time indicator appears to have gone missing. Solutions elsewhere for restoring it had no effect. So, in an effort to see if I could get an actual error message that might give me a clue, I tried running it from the command line and got:
$ /usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service ** (process:8092): CRITICAL **: Error parsing manifest for package 'com.ubuntu.calendar': com.ubuntu.calendar does not exist in any database for user [...] ** (process:8092): WARNING **: Unable to get snap information for 'com.ubuntu.calendar': Status code is: 404 Indicator-Datetime-Message: indicator-datetime exiting; failed/lost bus ownership
So. What do I need to either re-install, or delete from some ~/.config/ or ~/.local/ (or elsewhere) to reinitialize?