Why was Y2K specifically a big deal if computers actually store their numbers in binary? Why would a significant decimal date have any impact on a binary number?

Question

I understand the number would have still overflowed *eventually* but why was it specifically new years 2000 that would have broken it when binary numbers don’t tend to align very well with decimal numbers?

EDIT: A lot of you are simply answering by explaining what the Y2K bug is. I am aware of what it is, I am wondering specifically why the number ’99 (`01100011` in binary) going to 100 (`01100100` in binary) would actually cause any problems since all the math would be done in binary, and decimal would only be used for the display.

EXIT: Thanks for all your replies, I got some good answers, and a lot of unrelated ones (especially that one guy with the illegible comment about politics). Shutting off notifications, peace ✌

In: 478

Answer 1

The problem was not the modern binary representation or the technology in the 1990s in general. When computers began to be usable for reallife applications, every byte of memory was costly.

Software Engineers of the 1970s began to save as much resources as possible, and that included printing dates to paper for humans to read. One obvious pattern to save memory was to not have a second copy of identical dates (one that is human readable, and one that is binary), but to have number (and date) arithmetic operating directly on its human readable, decimal representation. It was a shortcut but it worked.

They were fully aware this solution would not work in the year >2000, but In the 70s no one expected their technology to still be around 30 years later.

But then of course working code gets rarely touched, to the contrary actually working code gets copied a lot. Such that old code easily ends up in banking backends, elevators, and what-not microprocessors.