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The dayOfYear accessor property of Temporal.PlainDateTime instances returns a positive integer representing the 1-based day index in the year of this date. The first day of this year is 1, and the last day is the daysInYear. It is calendar-dependent.
The set accessor of dayOfYear is undefined. You cannot change this property directly. To create a new Temporal.PlainDateTime object with the desired new dayOfYear value, use the add() or subtract() method with the appropriate number of days.
For general information and more examples, see Temporal.PlainDate.prototype.dayOfYear.