A fun fact about ISO 8601... it mandates[0] use of the proleptic Gregorian calendar for dates earlier than 15th October 1582. Now, in that calendar there is a y

A fun fact about ISO 8601... it mandates[0] use of the proleptic Gregorian calendar for dates earlier than 15th October 1582. Now, in that calendar there is a year zero - so anyone using the proleptic Gregorian calendar can say that the 3rd millennium began on 1st January 2000, instead of the pedant's preferred date of 1st January 2001. Since ISO 8601 is baked into so many modern systems and standards, and since software has eaten the world, there's a real sense in which we are all using it as our primary calendar - and thus for those of us who were celebrating on 1st January 2000, we can retro-canonically rest in peace knowing the fireworks were not mistimed after all. [0] OK, there's some fuzziness in the wording, where "mutual agreement" must be sought before exchanging such earlier dates.