Most common “Y2K-style” bugs today?

Daylight Savings Time issues, and other time-sensitive code snafus like one, will likely rear their ugly heads periodically for the foreseeable future:

https://issues.apache.org/SpamAssassin/show_bug.cgi?id=5852

Especially since the SA bug was patched to "match 2020 and onwards..."

When Unix time overflows...