18.104.22.168.9 Month state
The 'month' type doesn't allow for 13 month calendars (undecimber). The limitation to Gregorian months doesn't work here because you can't convert a month by itself to/from the Gregorian calendar.
What's the use case?
Here's a use case: A web site provides a calendar of events for a synagogue in Israel. The site groups events by month and lets the user pick the month to be displayed using an <input type="month"> element. Being the site of a synagogue, it has selected the Hebrew calendar by using the BCP 47 language tag he-IL-u-ca-hebrew. The Hebrew calendar is a lunisolar calendar with years of 12 or 13 months. This means, the input element needs to be able to handle up to 13 months.
This bug was cloned to create bug 17990 as part of operation convergence.