Metre Convention
ISO 8601:2004 fixes a reference calendar date to the Gregorian calendar of 20 May 1875 as the date the (Metre Convention) was signed in Paris (the explicit reference date was removed in ISO 8601-1:2019). However, ISO calendar dates before the convention are still compatible with the Gregorian calendar all the way back to the official introduction of the Gregorian calendar on 15 October 1582.