Had a quick look at the code and it seems it is checking if either KOrganizer or Kalendar are installed by checking for their respective .desktop files (org.kde.korganizer.desktop and org.kde.kalendar.desktop)
@carl Assuming the latter used to be what is now Merkuro Calendar then this check should probably be (additionally?) for org.kde.merkuro.calendar
The check is also just considering the “first” registered application for calendar data (app the user prefers for calendar data).
Might not hurt to check all offers if it wants to have one of two specific ones.
Since Merkuro Calendar has done the heavy lifting of creating QML API for accessing the calendar system, this might be as simple as
import org.kde.merkuro.calendar inside the digital clock applet and using the CalendarManager object from that module.
I.e. it could likely be done by anyone with Plasma/QML skills without having to dig into C++ PIM APIs.