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One can say that one event occurs after another event. Furthermore one can measure how much one event occurs after another. The answer to how much is the amount of time between the those two events. One way of defining the idea of 'after' is based on the assumption of causality. The work humanity has done to increasingly understand the nature and measurement of time, through the work of making and improving calendars and clocks, has been a major engine of scientific discovery.

The standard unit for time is the SI second, from which larger units are defined like the minute, hour, day, week, month, year, decade, and century. Time can be measured, just like other physical dimensions. Measuring devices for time are clocks. Very accurate clocks are often called chronometers. The best available clocks are atomic clocks.

There are several continuous time scales in current use: Universal Time, International Atomic Time (TAI), which is the basis for other time scales, Coordinated Universal Time (UTC), which is the standard for civil time, Terrestrial Time (TT), etc. Mankind has invented calendars to track the passages of days, weeks, months, and years.

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Time in engineering and applied physics In physics, time is defined as the distance between events along the fourth axis of the spacetime manifold. Special relativity showed that time cannot be understood except as part of spacetime, a combination of space and time. The distance between events now depends on the relative speed of the observers of the events. General relativity further changed the notion of time by introducing the idea of curved spacetime. An important unit of time in theoretical physics is the Planck time – see Planck units for more details.

See also: Synchronization, ISO 8601, Allan variance

Time in philosophy and theoretical physics

Important questions in the philosophy of time include: Is time absolute or merely relational? Is time without change conceptually impossible or is there more to the idea? Does time "pass" or are the ideas of past, present and future entirely subjective, descriptions only of our deception by the senses?

Zeno's paradoxes fundamentally challenged the ancient conception of time, and thereby helped motivate the development of the calculus. A point of contention between Newton and Leibniz concerned the question of absolute time: the former believed time was, like space, a container for events, while the latter believed time was, like space, a conceptual apparatus describing the interrelations between events. McTaggart[?] believed, rather eccentrically and on the basis of a very shaky argument, that time and change are illusions. Parmenides (of whom Zeno was a follower) held a similar belief based on a similarly shaky, but rather more interesting argument[?].

Einstein's theory of relativity linked time and space into spacetime in a way that also had philosophical consequences, making the idea of block time more credible, and thus affecting ideas of free will and causality.

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How to say times in English

  • Ask the time : what time is it? and whatīs the time?.
  • Answer: Itīs and the part of the hour or the minutes, seconds... Itīs five oīclock.
  • Parts of the day:
  • Parts of the hour
    • oīclock I.e. : itīs one oīclock (1:00).
    • Past and to:
      • Itīs 4 past 5 (5:04).
      • Itīs 20 to 5 (4:40).
    • Quarter:
      • A quarter past. I.e.: itīs a quarter past 9 (9:15).
      • A quarter to. I.e. : itīs a quarter to 12 (11:45).
    • Half-past. I.e.: itīs half-past 9 (9:30).
  • Answer to when? :
    • Certain time: At. I.e.: at 5 p.m..
    • Proximity:
      • Nearly, about : itīs about 10 oīclock.
      • Just after: itīs just after 10 (some minutes past 10).

See also: how to say dates in English.

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