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How do people know, say, that some specific event happened in China 1200 BC? Or the Mayans did something in AD 150? Back then, didn't they all use a different calendar system?

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    By tracing the different calendars backwards from a known, synchronised date. – Semaphore Apr 27 '15 at 1:35
  • @Semaphore Ok... but how do they know this date was synchronized? Do you have an example? – Jack Apr 27 '15 at 1:52
  • Web search: chronology. – Tyler Durden Apr 27 '15 at 2:01
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The term you're looking for is chronology (Wikipedia's article on the subject is rather sub-par).

In general, there are two major ways of synchronizing dates.

  1. Find an event recorded in more than one calendar. For example, if a treaty was signed on "a.d. VIII Kal. Oct, Julius et Caesar consulibus" and "7.14.19.11.14", you can establish an equivalence between the two and convert dates back and forth; a sufficient chain of conversions will let you figure out what the date would be in a modern system. In practice, you'll want multiple dual-dated events, because of errors in the historical record.

  2. Find an astronomical event that was recorded with a date. Total solar eclipses are especially good for this, since they're rare and only visible from a small area. If you've got even a "within a century or two" idea of how the calendar you're working with aligns to the modern dating system, there's usually only one eclipse that's a candidate. Again, you want multiple events to deal with possible error (eg. an annular eclipse that was recorded as total, or a "great comet" that was on a hyperbolic orbit rather than the short-period comet you thought it was).

  • Addendum: The astronomers use so-called Julian day (not to be confused with Julian calendar). It is also sometimes adopted as a reference where calendars are common, but not regular (eg. switch from Julian calendar to Gregorian with lack of 10 days in 1582, different leap years in Roman calendar etc.). – Voitcus Apr 27 '15 at 6:09

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