ISO 8601

YYYY-MM-DD

As for why it's better:

1. It's in sort order. Even if you use it for things that don't understand dates, they sort properly because the day is the last part, month is the middle part, and year is the first part. If you include hours, minutes or seconds, the sort order is still preserved.
2. Speaking of things that don't understand dates, because there are no slashes, you can easily use it in file names, directories, and anything else that treats slash as a special character. In addition, if you include the time part, because it uses "T" to separate the date and time, it doesn't use a space, so once again it can be easily used anywhere where a space might cause issues.

This is the way.

RFC 3339

ISO would be great, but sadly OmegaStar doesn't support ISO timestamps yet.

ISO 8601!

Yep. Doing that joke and landing on anything but ISO-8601 would be a huge red flag for me.

Being able to sort numerically and have it also be in chronological order is sooo cool.

Yep and it needs to use hyphens and not slashes. It helps avoid missed typos or mis-reads in some situations, especially if hand-written.

RFC3339 is better than ISO8601 since it doesn't allow some of the esoteric formats, and allows some useful formats that ISO8601 doesn't (like separating date and time by a space instead of by T)

For example, 2026-W32-1 is also a valid ISO8601 date: Monday on the 32nd week of 2026. That format isn't allowed with RFC3339.

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

nerd detected

Beat me to it, except I’d do yyyy-mm-dd because some OSes throw fits about having slashes in file names. Way better than the “Group Project-final FINAL v3 PAUL USE THIS ONE.file” versioning system that people seem to use.

Please use any kind of separation character or I'm gonna lose my mind

I like YYYY-MM-DD because then I don't have to figure out half the time if they used MM-DD-YYYY or if they were sane.

Ah yes, Mon/2026, perfect…