Parsing less than complete ISO representations

[mattjohnsonpint]

moment('2017-01') is undocumented in the supported ISO strings, but is correctly parsed as 2017-01-01 local time without giving a deprecation warning.

moment('201701') is the ISO 8601 basic form of the same representation, but this gives a deprecation warning, falls back to JS Date, and results in the entire number being interpreted as the year.

moment('2017') is also allowed under ISO8601, but we don't support it.

We should either fully support minimal representations less than a full date, or not support them at all, and we should document this either way.

[butterflyhug]

Are you sure that '201701' is a valid ISO basic representation? We have a comment in the code that asserts YYYYMM (aka '201701') is not part of the standard:

moment/src/lib/create/from-string.js

Line 22 in 7589bb8

// YYYYMM is NOT allowed by the standard

Assuming past-us is correct here, I think it would minimize confusion if we support and document '2017-01' and '2017', and explicitly document that '201701' was omitted from the standard.

[alburthoffman]

ISO 8601 does suport simply form '201701' and '2017'
we should fix this one, and add test cases for this.

[butterflyhug]

I repeat: are you sure about that?

I don't have a copy of the standard handy, but Wikipedia also asserts that year+month requires a hyphen alongside a plausible rationale:

By disallowing dates of the form YYYYMM, the standard avoids confusion with the truncated representation YYMMDD (still often used).

https://en.m.wikipedia.org/wiki/ISO_8601#Calendar_dates

[mattjohnsonpint]

You know what, you're right. I have a copy, and for year+month, the basic form is YYYY-MM and the extended form is listed as "not applicable".

Year-only is allowed though. It is at least positive four digits 0000 - 9999, but can support five or 6 digits and negatives "by agreement"

We only support positive four-digit years with YYYY-MM-DD, so I think if we add year-only recognition it should match.

To summarize:

• Document that we do support YYYY-MM as a recognized ISO8601 string
• Add support for YYYY and document that too.

[ghost]

Hello! It's my first time contributing to opensource. Can I take this? :)

[medinarkhov]

Hopefully #4470 will fix the issue