There's this handy venn diagram that I've seen floating around for a long time.

Just found a random link to it with an image search:

https://gyazo.com/d8517f72e24c38f055e17182842b991c/max_size/...

ISO 8601 does have some strange formats...

That's a screenshot of this website:

https://ijmacd.github.io/rfc3339-iso8601/

Thanks! That's where I remember it from.

> ISO 8601 does have some strange formats...

Not that I’ve ever really had cause to use it in anger, but I like the idea of ISO week dates.

Effectively, the ISO weak year (often differs from the calendar year in the last week of December / first week of January), ISO week number and day of week form a leap week calendar - instead of having 365 days in a common year and 366 in a leap year, it has 364 days in a common year and 371 in a leap week year, with leap weeks (obviously) being less frequent than leap days.

The Sym454 calendar [0] takes this idea further to create a perpetual calendar, with 12 months of 4 or 5 weeks; in leap years the 12th month is 5 weeks instead of 4 weeks long. However, the leap week rule proposed by Sym454 is different from that proposed by ISO 8601; the author of Sym454 argues his proposed rule has theoretical advantages (simple calculation and more uniform distribution of leap weeks). That said, there is a variant of Sym454 which uses the ISO 8601 leap week rule.

[0] https://kalendis.free.nf/symmetry.htm?i=1

I believe two-digit years haven't been allowed for a while:

> ISO 8601:2000 allowed truncation (by agreement), where leading components of a date or time are omitted. Notably, this allowed two-digit years to be used as well as the ambiguous formats YY-MM-DD and YYMMDD. This provision was removed in ISO 8601:2004.

(That's from https://en.wikipedia.org/wiki/ISO_8601 - I don't have the standards handy, ironically.)

Honestly I'm happy with either the RFC or ISO, but it seems like most normies haven't heard of RFCs so ISO is my default.

I believe you. But I haven't paid ISO for the pleasure of reading the actual spec so that passed me by.

Plus RFC3339 allows you to use a space instead of the ugly T delimiter between the date and time.

> 6 digit years

Totally insufficient for capturing important future events like the dead of the sun.