From 1997 through 2002, I stayed busy helping many businesses convert from computer accounting software that was not Y2K compliant.
To refresh your memory, when the calendar turned over to January 1, 2000, a number of older software codes failed to work properly. This was because a lot of programs used a six-character date, representing the year in two digits.
This was done to save the costs of data storage; I know, seems funny now. This practice worked great until we cycled to zero-zero. Then, not so good.