Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization.These calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year.Periodic events and periodic motion have long served as standards for units of time.Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart.
Time is used to define other quantities—such as velocity—so defining time in terms of such quantities would result in circularity of definition.
Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but it is now the most commonly used calendar around the world, by far.
During the French Revolution, a new clock and calendar were invented in attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar.
One view is that time is part of the fundamental structure of the universe—a dimension independent of events, in which events occur in sequence.
Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.