Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Ancient cultures such as Incan, Mayan, Hopi, and other Native American Tribes – plus the Babylonians, Ancient Greeks, Hinduism, Buddhism, Jainism, and others – have a concept of a wheel of Time: they regard Time as cyclical and quantic, consisting of repeating ages that happen to every being of the Universe between birth and extinction.
Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago. The reforms of Julius Caesar put the Roman World on a solar calendar. Pope Gregory XIII introduced the Gregorian calendar in in 1582. The most accurate Time keeping devices are atomic clocks, which are accurate to seconds in many millions of year. Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize Time keeping systems across the World.
Two distinct viewpoints on Time divide many prominent philosophers. One view is that Time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. An opposing view is that Time does not refer to any kind of actually existing dimension that events and objects ‘move through’, nor to any entity that ‘flows’, but that it is instead an intellectual concept (together with space and number) that enables humans to sequence and compare events.
A modern philosophical theory called presentism views the past and the future as Human-mind interpretations of movement instead of real parts of Time (or ‘dimensions’) which coexist with the present. This theory rejects the existence of all direct interaction with the past or the future, holding only the present as tangible.