RFC 3339! ISO 8601 has way too many weird formats that are allowed like today would be 2023-W41-2. See for example here.
RFC 3339 vs ISO 8601

Comparison between RFC 3339 and ISO 8601 date formats

Whoa, that’s a cool website!
It’s really pleasing seeing the seconds all change in unison!
I feel offended - W%W-%w is my preferred way of noting down dates :D
Great, now I need to memorize β€œRFC 3339”, because I officially have a new favorite date format. Thank you!

Fortunately this one is easy:

three threes equals 9 3339

You say that’s weird. As a programmer, this seems insanely useful
RFC 3339 when you need the basics, ISO 8601 when you need something more niche. Some applications genuinely need to view the year as weeks and days of the week instead of months and days of the month.