Since the upgrade to macOS Sierra I occasionally see a service CalNCService running hot (i.e. on 100% CPU). So going from the name, this seems to be some Calendar Service. I tried disabling Calendars in Internet Accounts Preferences, but this was no help. One can kill the service or restart the computer, but the problem persists. Usually, after a few hours or so it goes back to normal.
What is this daemon doing? How can I debug this issue?
Update: In the log console I find this when calNCService starts up:
warning: dynamic accessors failed to find @property implementation for 'serialNumber' for entity ABCDAddressBookSource while resolving selector 'serialNumber' on class 'ABCDAddressBookSource'. Did you remember to declare it @dynamic or @synthesized in the @implementation ?
Update2: The Service is connected to syncing reminders. Disabling reminder sync under Internet Accounts makes the service go back to normal, enabling it again makes it go back to 100%. This seems independent of the server. I have a bunch of recurring reminders and first had this problem with my account on fruux.com. Then I migrated all to iCloud and now its the same there.
Update3: Still very present in 10.12.1. and no reply on Apple Support Communities.
Update4: Another morning, another calNCService. I have also manually checked the exported .ics of the offending reminder list. It looks totally harmless. About 40 Reminders with nothing special in them. 
Update 5: Still all the same in 10.12.2
Update 6: Confirmed in 10.12.4. Iterating: This seems to be some housekeeping task. When I activated the reminders after the upgrade the problem was not triggered for more than 24h. But now it is back. As before, it can be simply stopped by deactivating reminder sync under "Internet Accounts"
Update 7: The bug seems to be still present in MacOS 10.13 High Sierra, it is less severe, though. I have been letting the corresponding reminders account activated. During an uptime of 13 days CalNCService clocked in a CPU time of 1:07:01 (31h), way too much, but less than e.g. "kernel task" (7:16:16).
Update 8: Still present in 10.13.4. I have two machines with MacOS and it happens on both of them. It seems to be connected to reminders added via some API such when you add reminders via Launchbar. The CPU time of calNCService seems to be correlated with how much I use Launchbar or Siri to enter reminders.
Best Answer
I had same problem (fans at full throttle, CalNCService at 122% of CPU) and have made 2 visits to Apple store and 3rd visit I checked in the Mac for a warranty repair (replaced both fans)... Yet the problem continues.
I went into notifications > edit and disabled all notifications, badges, sounds, etc, except for iMessage, Slack and Outlook.
About 3 minutes later, I noticed.... silence. Fans are normal. I'll watch it for a while, but it seems this issue is definitely related to something in notifications. I won't miss being pinged all day long.