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

They didn’t store the year in binary. Y2K problems typically involved a technique called [binary-coded decimal](https://en.wikipedia.org/wiki/Binary-coded_decimal), or BCD. Computers were *much* slower then, and spending dozens of CPU cycles on a division (by 10) just to display a number was considerable.

BCDs persisted even in microprocessors like the Intel x86 and Motorola 68000 (used in the Apple Macintosh computer and others), but they were not used to the same extent as on business mainframes.