The Númenórean calendar was devised during the Second Age, but even after the fall of Sauron at the end of that Age, the old calendar was maintained by the Exiles of Númenor in Middle-earth into the Third Age. The change of Age, though, introduced an error into the calendar that its designers had not foreseen, so that eventually a thorough revision became necessary. That revision was introduced by Mardil Voronwë, whose Revised Calendar consisted of twelve months, each of thirty days, and five additional days that belonged to no month. This Revised Calendar, more commonly called the Stewards' Reckoning, was introduced nearly a thousand years before the War of the Ring, and became the standard calendar of Middle-earth.

For a detailed breakdown of the Revised Calendar, with modern equivalent dates, see the entry for the Stewards' Reckoning.