Actually, almost no country outside the British Empire (and not even Scotland inside it) implemented the Gregorian reform at the same time. Britain was pretty late in doing so, too.

As for "all modern computer systems", Unix time is completely ignorant about anything except seconds.

> Unix time is completely ignorant about anything except seconds

That's not correct, actually: UNIX time tracks UTC instead of TAI meaning it "corrects" for leap seconds. As a result, UNIX time is not "the number of seconds since epoch" but "86400 * (number of whole days since epoch) + (number of seconds since midnight)", and UNIX time will go forwards (never so far) and backwards on leap seconds (a second will repeat in most implementations as the day goes from 23:59:60 to 00:00:00, as they have the same timestamp).

Except it doesn't account for leap seconds.

Is that true? Isn't that the responsibility of the whatever component that translates the number of seconds since Jan 1 1970 into a calendar?

POSIX requires that the Unix timestamp value be incremented by exactly 86400 per day, regardless of how many seconds that day actually contained.

https://en.wikipedia.org/wiki/POSIX_time#Encoding_time_as_a_...

Unix time is not the number of seconds since Jan 1st 1970.

> Unix time is completely ignorant about anything except seconds.

In hindsight, it seems like a brilliant decision.

But then we had to go and make it care about leap seconds...