I just saw a glitch in the update to Sierra from El Cap. All the events that I had entered into Calendar were missing. Fortunately, I had made a clone of the El Cap system before updating so I was able to boot into it and retrieve the entries.

How did this happen?

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365