Most cultures of Middle-Earth used a system of a seven-day week, the Elves however, being immortal used a basic unit of counting time known as the yén or "great Year", corresponding roughly to 144 löar or coranari, sun-rounds or Sun-years.The Edain originally had adapted the Calendar of the Sindar, their númenórean descendants however later modified the elven calendar of Beleriand to their needs, eventually also drawing from earlier pre-beleriandic customs among men.With the rising of the Sun the mortals had started to calculate time with the rounds of the sun and invented a primitive solar-calendar with a year of approximately 365 days divided into twelve "Sâti" or months of each 30 days, each Sât divided into four "Otglâtai", weeks of seven days each.

The Elves in Imladris (Rivendell) divided the solar year into six "seasons" or "long months", the Númenoreans established the average length of a year as 365.24 days (or more accurate tropical year of 365.2422 days) while the average year in the later 'Reckoning of Rivendell' was of ​52595⁄144≈365.24306 days. To attune the solar year to the moon-year the Dúnedain established leap-years every fourth year except centennial years and added to some years a leap-day known as "Cabed" or "Orlinda" (W. "Overlithe).


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