We programmers worked like dogs with the Y2K code mods. It was all hands on deck. Dates were often stored internally as YYMMDD integers for sorting purposes. Remember storage was at a premium in the explosive days of the 1980s and beyond. I wrote a ton of code fixes... Miserable and boring as cutting and stacking cord wood.
The early hack was, any stored date with year 49 to 99 was considered 1900's and 00 to 48 was to be treated as 2000's. Something like that. Luckily the IT world did not fall off the end of the world...