A calendrical system for measuring absolute time devised in Mesoamerica during the Formative (Pre‐Classic) period. The day forms the basic unit (kin), thereafter multiplying up into larger units progressing by 20s: uinal (20 days), tun (360 days), katun (7200 days), and baktun (144 000 days). Dates are expressed as units in a five‐place notation starting with the largest units. Thus 18.104.22.168.1 means 8 baktunob, 2 katunob, 4 tunob, 5 uinalob, and 1 kin. The Maya began their counting at a time equivalent to 3114 bc, perhaps marking some mythical event in Maya history. They inscribed their dates in the notation given above as hieroglyphs in which bars represented units of five and dots units of one. The long count system was replaced after ad 900 by a short count system, in which the katun replaced the baktun as the largest unit.