Late 2013 i7 iMac (non-retina), shortly after I upgraded to Sierra I noticed the Date & Time function had stopped and that attempting to correct the issue in Preferences resulted in an error message: "Could not load Date & Time preference pane." My first thought was selecting "Allow your Apple Watch to unlock your Mac" was the culprit. I unchecked the box, rebooted (which, incidentally, clears the issue), and waited to see what would happen. Within a week the Date & Time had stopped again. Another symptom is menu bar apps like bluetooth status, wi-fi, eject, volume, character launcher, active user, search, Siri, and notifications are inoperative; moving the cursor to the menu bar at one of them generates a spinning beach ball. I looked at the Web sites of all the third-party apps installed that use Date & Time data. None of them mention anything about this issue. I've checked sites where the gurus live, including Apple, and found no mention of the issue. Rebooting clears the issue until the next time, which could be a week or a day. Other than not knowing where along the time continuum it is, the iMac works fine. But the freaking clock outta work, ya know? I'd appreciate any thoughts all y'all might have about this issue. Thanks!