There were several reasons. One, everybody used two digit years for everything. When writing checks you used two digits. When you saw a date on television or in a book it used a two digit year. So it just made sense.

Secondly there was the idea that it would save space. And really it did. When your computer new out of the box had 3.5 KB available storage, those two extra characters were very important.

And thirdly because computers were new and things were changing so rapidly that everybody assumed everything would be replaced in just a few years. It's a bit difficult to explain, but electronic data was considered a lot "less real" then than it is now. When you switch the computer off it's gone. The idea that digital data would persist for years was difficult to grasp for a lot of people. Floppy disks had a realistic lifespan measured in months and later in years, but single digits for sure. It wasn't until optical drives became commonly available that it was possible for digital personal data to last very long at all.

Thanks for your reply. This is an interesting psychological perspective (the other comment offered a technical one) and it makes a lot of sense.

To expand on another good comment: space was precious. Really precious. If you were only a kid in the 90s you will have a hard time appreciating it. (But then again ancient UNIX greybeards will say the same to me, I guess...)

For example, my first computer was Apple II. It had 48KB RAM and 12KB ROM which contained initial bootstrap code that corresponds to BIOS, plus a Basic interpreter, plus a REPL tool with disassembler. All of that, in 12KB. The main page of Hacker news is about 42KB, not including icons and HTTP headers: it's already about three times a BIOS+Basic interpreter+disassembler.

And the monitor only had 40x24 lines (or maybe 40x25, it was a long time ago). Nobody's mad enough to waste two columns, out of 40, just to print two digits that will be forever stuck at 19.

As another example, around that time Turbo Pascal was immensely popular, and every string started with a byte showing its length: so no string can be more than 255 bytes. Good enough: what kind of madman would waste precious memory on a string more than 256 characters?

If you go back to that period and think about the environment, using 4-digit years will be considered a madness, not the other way.

Thank you very much for your reply. I definitely lack perspective on this. I really sympathize with the reasoning behind not printing a 4-length date on the low column displays.

But I’m still curious about just how wasteful a 4 digit year would’ve been to use internally (not to display). Wouldn’t it only use like extra 2 bytes of memory or less per “date” structure in the memory?

I don't have much personal experience in the matter, but I believe it's the mentality of working with very constrained systems, in this case limited memory, storage, processing. It was/is considered efficient and elegant, to shave off any possible unnecessary space or operations.

Those extra 2 bytes meant and "cost" a big deal more than they do now. At least, that's what I imagine is a reason behind the decision to use 2 digits to represent a year.

Future programmers will be asking us too, why we thought it was a good idea to use signed 32-bit binary integers to represent time - when we should have known it would run out soon.

https://en.wikipedia.org/wiki/Year_2038_problem

In earlier days of computing, storage was expensive. Saving two bytes for each date field multiplied by thousands of records across hundreds of files saved real money.

Another example of contortion for storage saving was Packed Decimal used in mainframes, where a (say) 11-digit number could be stored in 6 bytes rather than 11. There were even IBM 360/370 assembler math operations for packed decimal so they didn't have to be decided and re-encoded for arithmetic use.