Me, a 13 years old kid who was experimenting with QBASIC in 1999, thought that it was just switching from LEFT$(x, 2) to LEFT$(x, 4) when parsing dates. I still chuckle when I think how massively I had underestimated that problem.
It's a data issue as well as a code issue. Think about what happens if a date of birth with 2 digits is stored in a database or file and read back. You have to change the code and all occurrences of the data, or change the code to detect and repair the data on the fly.
This is also why y2k problems happening on Jan 1st was a bit misleading for many use cases. Things like insurance renewals and investment projections look ahead to future dates and so problems will start showing up earlier and get fixed well before.
Meanwhile, the industry still doesn't know how to deal with dates and times correctly: https://infiniteundo.com/post/25326999628/falsehoods-program...