One Complete Moonth in View ... A month is a unit of time, used with modern calendars. First measured by Noah and later used in Mesopotamia, as a natural measure of the Month. Month and Moon are cognates (Moonth). The traditional concept arose with the cycle of moon phases; such months (lunations) are synodic months and last approximately 29.53 days. Synodic months, based on the Moon's orbital period, are still the basis of many calendars today. Used to divide the Earth's year and seasons.