| Epoch (reference Date) |
Article Index for Epoch |
Information AboutEpoch (reference Date) |
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CALENDARS Each Calendar Era starts from an arbitrary epoch, which is often chosen to commemorate an important historical or mythological event. For example, the epoch of the current civil calendar is the traditionally-reckoned year of the birth of Jesus , defined as Year Number 1 . Thus, the first instant of January 1 , 2006 CE should be exactly 2005 years since the epoch, but quirks in the development of the modern Gregorian Calendar make this technically incorrect. The traditional Chinese Calendar uses 2637 BCE, a date in the life of the legendary Yellow Emperor , as its epoch. Several Other Calendars are also currently in use, based on important historical events. ASTRONOMY See Also: Epoch (astronomy) In astronomy, an epoch is a moment in time for which Celestial Coordinates or Orbital Elements are specified. The current standard epoch is J2000.0 . COMPUTING In Computer s, time is often expressed as the number of seconds since midnight, Universal Time , on a conventional epoch defined by the Operating System . Contrary to human calendars, computers usually start counting from 0 at the epoch instant. Famous epoch dates include:
System time is measured in seconds or ticks of arbitrary length past the epoch. Unspecified Problems may occur when this number exceeds a predefined capacity, which is not necessarily a rare event; on a machine counting 10 ticks per second, a signed 32-bit count of ticks allows for only 6.8 years of accurate timekeeping. The 1-tick-per-second clock of Unix will Overflow on January 19 , 2038 , creating the Year 2038 Problem on systems that still store time as a 32-bit signed integer. David Mills, author of NTP , acknowledges that the protocol's ultra-precise 64-bit timestamps will roll over on February 6 , 2036 and advises that: Should NTP be in use in 2036, some external means will be necessary to qualify time relative to 1900 and time relative to 2036 (and other multiples of 136 years). The evolving definition of official time over history introduces more subtle problems for computer-based linear representations. Leap Years and the Gregorian Calendar are generally taken into account, but Leap Second s are more challenging due to their non-linear rate of past occurrences and the impossibility to accurately predict their future occurrences. These complications are discussed at length in the Unix Time article. Trivia
January 1, 1904, was chosen as the base for the Macintosh clock because it was the first leap year of the twentieth century. {Link without Title} This means that by starting with 1904, Macintosh system programmers could save a half dozen instructions in their leap-year checking code, which they thought was way cool. EXTERNAL LINKS |
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