Previously I have used Ubuntu 12.04 LTS with MATE from your PPA.
It had indicator-datetime and shown calendar events in Clocks applet.
Similar example is shown in this AskUbuntu Q&A about ordinary Ubuntu:
Currently I have configured Evolution to access my CalDAV. I can execute /usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service, but I have no integration with MATE clock-applet.
Is it possible to get the same functionality on Ubuntu MATE 16.04 LTS?
You can use the same indicator on 18.04, the MATE clock applet can be replaced with the DateTime indicator as follows:
Unfortunately, I don't believe 16.04 has the necessary dependencies for the DateTime indicator to work, nor do I believe the MATE clock applet has this functionality either.
I have partially checked this method on 18.04 LTS. It shows two clocks (first from indicator-datetime with calendar and second - MATE Clock Applet).
Currently trying to make it working on 16.04 LTS. But simple installation of indicator-datetime from bionic to xenial does not work because of missed dependencies.
Can't get indicator-datetime working on 16.04 LTS even with MATE for Xenial PPA. It is not shown on MATE Panel after execution of /usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service (does not matter from terminal or by XDG Autostart).