> 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.