There is something called a Y2K22 bug in some software.
@bdw - did you have to get up and patch servers?
https://techcommunity.microsoft.com/...s/ba-p/3049447This wasn't due to a change on 31st Dec. The problem is caused by an integer overflow error: the anti-malware component is converting the date/time into "YYMMDDHHMM" format and storing it as a signed 32-bit number (max value 2147483648). So, in Dec 2021, the number would start with "2112..." (below the threshold). In Jan 2022, the number would start with "2201..." (above the threshold).