I think in 2001 I was making a Linux from scratch system having not gotten enough from red hat and Debian with home configured and compiled kernels

Fun times and no, nothing like the commercial home operating systems back then

One program I tested went from (31,12,99) to (01,01,100). Its front end formatted the date and added the century, so it showed 1 January 2000 as 01/01/19100

That wasn’t fixed. The fault didn’t affect processing (the years were wrong but had the correct offset between them) and was only visible to internal users, and also that system was expected to be retired in 2004