Date without year
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ISO8601 also supports dates without years which is a convenient way to represent birthdays or holidays: --12-25
It would be a good addition to this comparison.
ISO 8601:2000 Allowed a month and day without a year in what was known as Truncated representations. These formats omitted some of the most significant digits.
Ref: ISO 8601:2000 § 5.2.1.3 Truncated representations
Allowed examples included:
- YY-MM-DD
- -YY-MM
- -YY
- --MM-DD
- --MM
- ---DD
- -Y-Www
- -Www-D
- -DDD
However, all the truncated representations were removed in ISO 8601:2004.
Reduced precision formats are still allowed i.e. Omitting some of the least significant digits.
Ref: ISO 8601:2004 § 4.1.2.3 Representations with reduced accuracy
The diagrams get complicated if you try to add extra circles for the various editions of the ISO standard. I'm just aiming to represent ISO 8601:2019 right now. I might consider adding an options for users to compare historical versions in the future.
Just for the record, EDTF Level 2 in ISO 8601-2 adds X
as an Unspecified Digit in all places, so XXXX-12-25
would be a replacement for the initial example. (Level 1 only supports replacement of the rightmost components.)