Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The eraYear accessor property of Temporal.PlainYearMonth instances returns a non-negative integer representing the year of this year-month within the era, or undefined if the calendar does not use eras (e.g., ISO 8601). The year index usually starts from 1 (more common) or 0, and years in an era can decrease with time (e.g., Gregorian BCE). era and eraYear together uniquely identify a year in a calendar, in the same way that year does. It is calendar-dependent.
The set accessor of eraYear is undefined. You cannot change this property directly. Use the with() method to create a new Temporal.PlainYearMonth object with the desired new value.
For general information and more examples, see Temporal.PlainDate.prototype.eraYear.