Wikipedia says "ISO 8601:2004 established a reference calendar date of 20 May 1875 (the date the Metre Convention was signed), later omitted from ISO 8601-1:2019." I was curious what "reference calendar date" is supposed to mean.

Thanks to links in the SE thread, I found the relevant actual text in ISO 8601:2000 (I don't know how different it might be, if at all, in the 2004 document):

> The Gregorian calendar provides a reference system consisting of a, potentially infinite, series of contiguous calendar years. Consecutive calendar years are identified by sequentially assigned year numbers. A reference point is used which assigns the year number 1875 to the calendar year in which the “Convention du mètre” was signed at Paris.

This last sentence is simply an obtuse way to say "this year right now, as I [jomar] write this -- we call this 2025". Apparently the ISO committee did not want to refer to what was going on around 1 AD or felt that the missing 0 between 1 BC and 1 AD would lead to confusion or something, so instead used the birth year of the metre to state the bleeding obvious.