Thought you might be interested in this discussion at enwp, which will discuss how we implement custom date formatting based on Citoid's choice to use ISO ymd
Topic on Talk:Citoid
Would it be possible to get an official answer to some of the questions raised there?
Date ranges in ISO 8601-like format are not currently supported by WP:DATESNO so are not supported by cs1|2. If Citoid needs to specify a date range will it use the ISO 8601 format? If Citoid needs to specify a season, will it use ISO 8601 format? Is there an ISO 8601 format for seasons? What about dates outside of the Gregorian calendar; does Citoid support those? In what format? In a conversion, how do we specify use of abbreviated month names?
@Mvolz may be able to tell you what the current and/or future plans are for date ranges (e.g., May–June 2015). DATESNO doesn't appear to address ranges at all. I'm not sure what "not supported by cs1" means; I hope that it doesn't mean "emits error messages if given an accurate publication date for a bi-monthly magazine".
On the question of Julian calendars, are you aware of any URLs that report publication dates that pre-date the creation of the world wide web by several centuries?
cs1|2 supports a subset of date formats allowed by the various sections of Wikipedia:Manual_of_Style/Dates_and_numbers#Dates.2C_months_and_years of which Wikipedia:Manual of Style/Dates and numbers#Ranges is a portion. DATESNO is a term often (mis)used to refer to the dates portion of that MOS page because it is a redirect to en:Wikipedia:Manual of Style/Dates and numbers#Unacceptable date formats. cs1|2 compliance with MOS is specified at en:Help:Citation_Style_1#CS1_compliance_with_Wikipedia.27s_Manual_of_Style.
cs1|2 do not support ISO 8601-like date-range-formats because MOS only supports YYYY-MM-DD single-date style. At present, cs1|2 will emit an error message if given a YYYY-MM-DD/YYYY-MM-DD range.
It is very common for periodicals to date issues according to season. How does Citoid report those dates? What about dates that are proper names? cs1|2 allows Christmas YYYY as a date because there are periodicals that use that date. How does Citoid report that kind of date?
I don't know how Citoid gets its dates. If Citoid only works with on-line publications where the date of the cited material is guaranteed to be Gregorian, then there is no issue. But, if Citoid can report dates in the Julian calendar then that may or may not be a problem.
(I've fixed the links above. I believe that problem was that you had used [[en:foo]] instead of [[:en:foo]] - i.e. interlanguage links instead of interwiki links. :-) For future reference, you can edit your posts (and do other actions) by clicking on the "..." icon at the top corner of each post (and the topic as a whole). You can also flip between wikitext and visualeditor, by clicking the icon in the bottom corner. Hope that helps.)
That didn't come out right. Apparently I can't edit it to fix it either. Flow is, I guess, not ready for prime time. What I really wrote is at:
Help_talk:Citation_Style_1#Automatic_ISO_conversion
on the English Wikipedia
Hope this works.