I had a credit card with an expiration date of December 2019, which I assumed meant it was good throughout the entire month. But a couple days before the New Year (Dec. 30th) I started getting notifications from all sorts of places that my card had expired and that I needed to update my payment information. I even got a "payment declined" notification from one vendor.
No worries, I got the replacement card about a month or so ago. I just updated the information and everything went through. But I was a little puzzled as to why the card was expiring a few days before the end of the month instead of being good through the entire month. And then today I saw this:
Why 2020 started on December 30th
That explains that.