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In chronology, an epoch is an instant chosen as the origin of a particular time scale. The epoch serves as a reference point from which time is measured. Days, hours and other time units are counted from the epoch, so that the date and time of events can be specified. Events that took place earlier can be dated by counting negatively from the epoch. Epochs are generally chosen to be convenient or significant by a consensus of the time scale's initial users.

Calendars Edit

Each calendar era starts from an arbitrary epoch, which is often chosen to commemorate an important historical or mythological event.

Some other eras were in official use in modern times or are still in use in several countries alongside the current international Anno Domini era.

Asian national eras Edit

Religious eras Edit

  • In Israel, the traditional Hebrew calendar, using an era dating from Creation, is the official calendar. However, the Gregorian calendar is the de facto calendar and is commonly used. Government documents usually display a dual date. According to Jewish tradition, the world was created in approximately 3761 BC, corresponding to the year 0 in the Hebrew calendar. Therefore, the date, as of Rosh Hashanah in 2006, is 5767 years since the Creation of the world. [1] [2]
  • In the Islamic world, traditional Islamic dating according to the Anno Hegiræ (in the year of the hijra) or AH era remains in use to a varying extent, especially for religious purposes. The official Iranian calendar (used in Afghanistan as well as Iran) also dates from the hijra, but as it is a solar calendar its year numbering does not coincide with the religious calendar.

For example, the epoch of the current civil calendar is the traditionally-reckoned Incarnation of Jesus.[3] Thus, the first instant of January 1, AD 2006 should be exactly 2005 years since the epoch, but incomplete information about how the traditional epoch was chosen, combined with quirks in the development of the modern Gregorian calendar make this technically incorrect.

The traditional Chinese calendar uses 2637 BC, 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 Edit

Main article: 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 Edit

In computers, time is often expressed as the number of seconds or days (including a fraction) 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). (quoted from RFC 1305)

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 seconds 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 Edit

January 1, 1904, was chosen as the base for the Macintosh clock because it was the first leap year of the twentieth century. [...] 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.
  • The fictional (or Julian) leap day February 29, 1900 in Microsoft Excel was introduced intentionally in order to maintain compatibility with then market leader Lotus 1-2-3. Designers of Lotus 1-2-3 had probably chosen this simplified behaviour in order to save some precious processing time and program space. For the rest of its time range 1900 - 9999 Excel uses the Gregorian calendar, hence e.g. there is no February 29, 2100.
  • Microsoft Excel (and Lotus 1-2-3) technically consider the epoch of December 31, 1899 as January 0, 1900 or a serial date of zero (consequently, December 31, 1899 cannot be used). January 0, 1900 can be processed and formatted in Excel Worksheets, just as any other date.

NotesEdit

  1. My Jewish Learning:Counting the Years
  2. Rosetta Calendar
  3. Blackburn, B. & Holford-Strevens, L. (2003). The Oxford Companion to the Year: An exploration of calendar customs and time-reckoning. Oxford University Press. Glossary entry for "Incarnation era", p. 881.

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