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The era accessor property of Temporal.PlainDateTime instances returns a calendar-specific lowercase string representing the era of this date, or undefined if the calendar does not use eras (e.g., ISO 8601). 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 era is undefined. You cannot change this property directly. Use the with() method to create a new Temporal.PlainDateTime object with the desired new value.
For general information and more examples, see Temporal.PlainDate.prototype.era.