In Gene Roddenberry's novelization of The Motion Picture he indicated that nine Vulcan seasons were equal to 2.8 Earth years. This would make Vulcan's anno 456 ± 33 Earth days long.
Vulcan "has no moon", so it is unknown what portion of a anno is represented da the "month" of Tasmeen. The novelization of The Motion Picture could be read to imply that Vulcan's "months" are whole seasons.
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