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> If you want to store only the year, why not store the years since 1900? That way in a single byte you can go all the way up until 2155.

You're making an assumption in that statement that there's a byte and it's eight bits in length. This was not always true. Punch cards didn't really work that way, and fair number of CPU's didn't either.

Early machines used various decimal systems and 6-bit characters were widespread also. This explains a bit of why the C language and descendants often have native support for Octal literals and Unix permissions are built around three bit groupings.

On a personal note, even in the very early 90's, I also remember using a CDC machine (a descendant of Seymour Cray's 6600) that supported 60-bit words with 6-bit characters. Pressure to move to 64 bit words with 8-bit bytes resulted in dual mode design that could be booted either way.

CDC...if it was like the CDC system I worked on back in the early 80s, the 6-bit chars (64 distinct chars, including control chars) did not include lower case Latin. Our system was munged so that lower case alphabetic chars could be represented by prefixing a backslash. I wrote my dissertation on that system, and fortunately the editor I used displayed a backslashed char as lower case.

This is a good question. I found a copy of Fujitsu COBOL 85 manual [1] (the next release was post y2k, in 2002) and was surprised to see that CURRENT-DATE, INTEGER-OF-DATE use 4-digit years.

However some functions from that time (eg ACCEPT, p. 544) do use 2-digit years. I think you have it exactly right, it was about facilitating string formatting/display: ACCEPT took input from the terminal screen. (I think it is, approximately, gets() + strptime(), though I don't see any talk of error handling). Programmers probably just stuffed the result into a record before writing to DASD (disk), optimizing for programmer time and program complexity over storage.

Perhaps 2-digit years for ACCEPT were an optimization for data entry workers?

A modern z/OS COBOL guide [2] has 4-digit years throughout as far as I can see.

Whilst I'm here, I'm also going to say: "VSAM", "PDS", TSO", "LRECL" and "ABEND" (just because I haven't heard those words for a quarter century and now I'm feeling nostalgic).

[1] http://testa.roberta.free.fr/My%20Books/Mainframe/Fujitsu%20...

[2] https://www.ibm.com/support/knowledgecenter/SS6SG3_4.2.0/com...

What about "JCL" ? Those ABENDs don't just show up on their own.

When 5MB disk storage system is the size of a small car and costs $50k, you fight for bytes. Or if your data is stored on magnetic tapes and reading in a dataset can be costed by the kilobyte, takes minutes per megabyte, and requires a million dollar's worth of equipment and dedicated staff, you fight for every byte.

The phone I carry today is hundreds or thousands of times more powerful than the $5,000,000 mainframe I was working around Y2K and the apps on it - most delivered freely - make it far more capable than the raw processing power difference would indicate.

If you are fighting for bytes then storing dates as "DD-MM-YY" strings is very inefficient, as you are wasting a factor 4X on storage compared to just storing the days since 1900 as a 2-byte integer. I understand that storage used to be expensive, but I don't understand where the two-digit year comes into play as I can't envision an efficient storage mechanism that is limited to exactly the years 1900-1999.

You need also to have the code to do the conversion. If you store, in a byte, the years since 1900 (or 1970) then every time you load and/or process a date to be displayed you need to add 1900 (or 1970) and then convert from int to str which takes time (machines were slow) and code.

And the code takes space.

You may think the space is trivial, but I remember working for nearly two weeks on a commercial system to save around 15 bytes on a system, and it was worth it.

Then the culture was embedded ... you worked to save bytes, and didn't do things that would cost more bytes. And the systems worked, and didn't need updated, so they persisted.

BCD (Binary-coded decimal) comes to mind; which can store values 0-99 in one byte and with native support in older CPUs.

One of my tasks for Y2K prep was to modify an in-house-developed record-based "database" to accommodate a the century change. Originally developed in the '60s the year had originally been a single digit because everyone thought commercial DBMS' would be viable in few years so there would be a replacement before the decade rolled over. :-| They'd grafted decade handling into the thing before I got there so it could survive the '70s because they had a lot of expensive data in various file sets that contained the data. Converting to a commercial database was expensive and represented a huge business risk, so...

If you have eight bit bytes, you would of course store these as three two digit BCD numbers requiring one byte each, and need no conversion logic. (The code for printing BCD numbers is already there, because you need it to print amounts of money anyway).

A whole byte for a year? What a waste in 1995 when you've got just 640KB for DOS, your code and your data.

I looked up my old code. Shave off a bit of the year and:

  struct Date { 
       unsigned day : 5; 
       unsigned month : 4; 
       unsigned year : 7; 
    };

a full date in 16 bits. (well, in borland C++ 5 at least)

and as it was an accounting application, working with the bitfields was so much nicer than ordinal days you would use nowadays.

month and years was all you really cared about anyway when summarising data. in finance all months have 30 days anyway.

the problem came from the 60's when memory was really expensive on mainframes and you had things like drum storage

> why not store the years since 1900

Most input systems were punch cards, which were designed to be fixed-length textual input directly from a user at a card punch machine. Teaching users how to map from “years since 1900” to the correct keys on the card punch is a lot more confusing and error prone than having the computer automatically convert from "71" to 1971.

That makes a lot more sense, but if it is just input then why not do the conversion of the string "71" to the efficient byte representation of 1971 when it is moved into the storage. If the computer can do a mapping of "71" -> 1971 then why can't it also do a mapping of "10-10-71" -> 26214 (days since 1900-01-01).

Then you would need another column on the card, and there are even fewer columns on a card than there are bytes in RAM.

The punched card, for a long time, _was_ the storage.