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The epochMilliseconds accessor property of Temporal.Instant instances returns an integer representing the number of milliseconds elapsed since the Unix epoch (midnight at the beginning of January 1, 1970, UTC) to this instant. It is equivalent to dividing epochNanoseconds by 1e6 and flooring the result.
The set accessor of epochMilliseconds is undefined. You cannot change this property directly. To create a new Temporal.Instant object with the desired new epochMilliseconds value, use the add() or subtract() method with the appropriate duration.