Time has been understood differently by different cultures, by different philosophers and physicists, and in different contexts. This article will look at these differences in terms of three main approaches: the metaphysics of time, the perception of time, and the metric of time.
Extensive reviews are available  ;  on the philosophical and metaphysical aspects of time.
A concise review on the perception of time is available . A book  on time perception is also available.
Following the International System of Units (SI), the physical time is measured in seconds. A second is defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom at a temperature of 0 Kelvin" (, retrieved August 21st, 2007).
Many other measure units are in common use, which often do not obey the rules of the SI. Some common examples are:
- minute (min, 60 seconds)
- hour (hr, 60 minutes)
- day (d, 24 hours)
- week (wk, 7 days)
- month (mo, 28 to 31 days)
- year (yr, 12 months)
In geology, in order to describe even larger time intervals, other units are common as:
- kiloyear (ky, 1000 years)
- Megayear (My, 1 million years)
A further unit, Ma, is used to indicate elapsed time, thus, 1 Ma means one million years before present. It is common practice to set the present at year 1950.
- Raymond Flood and Michael Lockwood [edd] The Nature of Time. Oxford: Basil Blackwell, 1986.
- Alfred Gell The Anthropology of Time. Oxford: Berg, 1992.
- Peter J. King "Time", in H. James Birx [ed.] Encyclopedia of Anthropology Volume 5. Thousand Oaks: Sage Publications, 2006. ISBN 0-7619-3029-9
- Robin Le Poidevin and Murray MacBeath [edd] The Philosophy of Time. Oxford: Oxford University Press, 1993.
- J.M.E. McTaggart "The Unreality of Time". Mind 17, 1908, pp 457–484.
- D.H. Mellor Real Time II. London & New York: Routledge, 1998.
- ↑ Whitrow GJ. (1961) The Natural Philosophy of Time. Nelson, London.
- ↑ Whitrow GJ. (1988) Time in History: The Evolution of Our General Awareness of Time and Temporal Perspective. Oxford University Press. ISBN 0192153617 Full-Text Online with Subscription
- ↑ The Experience and Perception of Time (Stanford Encyclopedia of Philosophy). Retrieved on 2008-02-06.
- ↑ APA Digital Object Identifier. Retrieved on 2008-02-06.
- ↑ "Time Perception" by Edward Willett. Retrieved on 2008-02-06.
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