In addition to the calendar slot, @pipobscure suggested adding two new slots to Temporal.Date:

• isLeapMonth
• era

Should isLeapMonth be bundled into month? For example, a calendar could choose to store a record like { number, isLeap } into the month field. I have a slight preference for this model. The record would have to be immutable (until the records proposal moves forward, this could be done via an object with a prototype). The decision here should dictate tc39/proposal-intl-displaynames#55.

Do we need any extra slots for Temporal.Time?