Dates with the year stored as two digits only (say, 1995 was stored as “95”), which worked fine for things like comparisons (for example: “is the year in entry A before or after the year in entry B?”) which were just done by numerical comparison (i.e. 98 > 95 hence a date with a year ending in 98 is after a date with the year ending in 95), until 2000 were the year being store would become “00” and all those assumptions that you could compare those stored years as numbers would break, as would as all the maths being done on two digits (i.e. a loan taken in 1995 would in 1998 be on its 98 - 95 = 3rd year with that system, but in 2000 it would be on its 98 - 00 = - 98th - so negative - year which would further break the maths downstream with interesting results like the computer telling the bank it would have to give money to the lender to close the loan).
Ultimatelly a lot of work was done (I myself worked in some of that stuff) and very few important things blew up or started producing erroneous numbers when the year 2000 came.
![The problem solver [workchronicles]](https://startrek.website/pictrs/image/e2bf50ef-e9bb-4041-874f-ad9702addd4b.jpeg){kind=link}
Dates with the year stored as two digits only (say, 1995 was stored as “95”), which worked fine for things like comparisons (for example: “is the year in entry A before or after the year in entry B?”) which were just done by numerical comparison (i.e. 98 > 95 hence a date with a year ending in 98 is after a date with the year ending in 95), until 2000 were the year being store would become “00” and all those assumptions that you could compare those stored years as numbers would break, as would as all the maths being done on two digits (i.e. a loan taken in 1995 would in 1998 be on its 98 - 95 = 3rd year with that system, but in 2000 it would be on its 98 - 00 = - 98th - so negative - year which would further break the maths downstream with interesting results like the computer telling the bank it would have to give money to the lender to close the loan).
Ultimatelly a lot of work was done (I myself worked in some of that stuff) and very few important things blew up or started producing erroneous numbers when the year 2000 came.