Out of desperation we restarted the web server, IIS. The sun broke through the clouds, birds were singing, and all was right with the world. The box was no longer confused about transatlantic dates. We did nothing but restart IIS. It was very scary.
After additional testing we did the upgrade and everything is fine. Which reminds me of a Haiku:
Yesterday it worked.
Today it is not working.
Windows is like that.