[ruby-core:76546] [Ruby trunk Bug#12285][Assigned] Date.iso8601 does not properly handle partial date strings

Issue #12285 has been updated by Nobuyoshi Nakada.

Status changed from Open to Assigned
Assignee set to tadayoshi funaba

As I read the code, `Date.iso8601` allows to omit a month by '-', but not a day.
It feels like a bug to me, but uncertain.

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Bug #12285: Date.iso8601 does not properly handle partial date strings
https://bugs.ruby-lang.org/issues/12285#change-59779

* Author: John Backus
* Status: Assigned
* Priority: Normal
* Assignee: tadayoshi funaba
* ruby -v: ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]
* Backport: 2.1: REQUIRED, 2.2: REQUIRED, 2.3: REQUIRED
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> For reduced accuracy, any number of values may be dropped from any of the date and time representations, but in the order from the least to the most significant. For example, "2004-05" is a valid ISO 8601 date, which indicates May (the fifth month) 2004.

From https://en.wikipedia.org/wiki/ISO_8601#cite_ref-AccuracyVsPrecision_8-0

However if I parse this example value with `Date.iso8601` I get "January 5th, 2004" when I expect "May 1st, 2004":

```
$ irb -r date
2.3.0 :001 > date = Date.iso8601('2004-05')
 => #<Date: 2004-01-05 ((2453010j,0s,0n),+0s,2299161j)>
2.3.0 :002 > date.month
 => 1
 ```



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