Reckoning of Years
The Reckoning of Years was a book written by Merry Brandybuck sometime after the War of the Ring. In this work Merry discussed the relation of the calendars of the Shire and of Bree to those of Rivendell, Gondor, and Rohan.
"Reckoning of Years" is a rendering into English of De Temporum Ratione, the title of an important treatise on astronomy and timekeeping by the seventh-century English monk Bede. This work is the sole source of the Anglo-Saxon names of the months of the year, which Tolkien adapted for the Shire calendar.