 To approximate his tropical year Hipparchus created his own lunisolar calendar by modifying those of Meton and Callippus in "On Intercalary Months and Days" (now lost), as described by Ptolemy in the "Almagest" III.1 (Toomer 1984, p. 139).The Babylonian calendar used a cycle of 235 lunar months in 19 years since 499 BC (with only three exceptions before 380 BC), but it did not use a specified number of days.The Metonic cycle (432 BC) assigned 6,940 days to these 19 years producing an average year of 365+1/4+1/76 or 365.26316 days.