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Stewards' Reckoning, also known as the Revised Calendar, was the calendar system introduced in Gondor by its first Ruling Steward, Mardil Voronwë, in Third Age 2060. It replaced the previous system known as the Kings' Reckoning and, following the War of the Ring, it was replaced by the New Reckoning.
Arnor and Gondor used a Númenórean calendar called Kings' Reckoning 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. When the era of the Kings passed with the fall of Arnor and the loss of King Eärnur, the Kings' Reckoning presented (through the continued accumulation of the millenial deficits) an offset of about 1.6 days out of synchronicity with the astronomical observations.
To correct the accumulating deficit of the calendar, Steward Mardil inserted two leap days into T.A. 2059. In addition, he reformed the calendar, to take effect in 2060, by making all months of equal length at 30 days and arranging the two extra days as holidays outside of the months. In 2360 Steward Hador added 1 day to that year. However, there was no millennial addition in 3000, so by 3020 the calendar was once again in deficit according to Tolkien, but by less than 1 day.
It is unclear whether the leap day in 2060 was dropped in consequence. The deficit Tolkien would have calculated ignoring the additions in 2059 for the end of 2060 is 1 day, 8 hours, 5 minutes, 26 seconds; his claim that the deficit then was about 8 hours would imply that one extra day had occurred and therefore that the leap day was skipped. By contrast, his claim that a deficit of less than one day existed in 3020 would imply that the 2060 leap day as retained.
 The Calendar
The Revised Calendar consisted of twelve months, each of thirty days, and five additional days that belonged to no month. The months were taken from those of the Kings' Reckoning, with the change that the seventh and eighth months were shortened to thirty days, and the two days placed outside the months as tuilérë, meaning Spring-day, and yáviérë, meaning Autumn-day.
- Yestarë: Winter Solstice
- 28 Gwaeron: Vernal Equinox
- 30 Gwirith: Erukyermë
- Loëndë: Summer Solstice/Erulaitalë
- 10 October: Automnal Equinox
- ?? Eruhantalë