The calendar year in my game jumped ahead by 10,000 years, despite the fact it's still set to Gregorian and still says CE.

Steps to reproduce:

I'm not 100% sure when this happened, but it happened some time after I turned off 'Enable mandatories' in the settings. When I did this, I also clicked on the Calendar setting to open the drop-down list, but I didn't change to the Holocene calendar. I noticed the bug about 5-10 turns after this.

When I switch to the Holocene calendar now, it says 20336 HE, and when I change it back it goes back to 10366 CE.