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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.
Each calendar era starts from an arbitrary epoch, which is often chosen to commemorate an important historical or mythological event.
Asian national eras
- The official Japanese system numbers years from the accession of the current emperor, regarding the calendar year during which the accession occurred as the first year.
- It is still very common in Taiwan to date events via the Republic of China era, whose first year is 1912.
- North Korea uses a system that starts in 1912 (= Juche 1), the year of the birth of their founder Kim Il-Sung. The year 2007 is "Juche 96". Juche means "autarchy, self-reliance".
- In Thailand in 1888 King Chulalongkorn decreed a National Thai Era since founding of Bangkok on April 6, 1782. In 1912, the New Year's Day was shifted to April 1. In 1941, Prime Minister Phibunsongkhram decided to count the years since 543 BC. This is the so-called Thai solar calendar or Thailand Buddhist Era clearly relied on the western solar calendar. This is one of the versions of the Buddhist calendar.
- 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.  
- 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. 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.
- 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.
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:
- January 1, 1 - Symbian epoch (using microseconds) and Microsoft .NET's DateTime epoch. Also used as base date in REXX counting days. This epoch is known as Rata die(rum).
- January 1, 1601 - Windows' Win32 FILETIME epoch (using 100-nanosecond ticks)
- November 17, 1858 - VMS epoch and the base date of the Modified Julian Day used in celestial ephemerides by the United States Naval Observatory and other astronomy organizations.
- December 31, 1899 - Microsoft Excel epoch, using the Julian calendar leap year rule for 1900 (hence with leap day February 29, 1900) and the Gregorian calendar for the years 1901 - 9999 ; thus for dates from 1 March 1900 a time is stored as the number of days in the Gregorian calendar from December 30, 1899, 00:00 (see also Trivia below); optionally Microsoft Excel can also use the Apple Macintosh epoch, which avoids the complication by starting later; it represents times by a number that is 1462 less.
- January 1, 1900 - Network Time Protocol epoch
- January 1, 1904 (local time) - Apple Macintosh epoch, through Mac OS 9. Palm OS epoch.
- January 1, 1970 - Unix epoch, Mac OS X, Java.
- January 1, 1978 - AmigaOS epoch 
- January 1, 1980 - MS DOS, OS/2, and other environments supporting a FAT file system encoding dates from 1980 up to 2107 in 16 bits.
- January 6, 1980 - Qualcomm BREW  and GPS  epoch
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.
- According to Martin Minow,
- 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.
- RFC 1305, defining the Network Time Protocol, includes a lot of information on time standards in historical and modern calendars, starting on page 81 (numbered 74) of the original specification in PostScript format.
- Critical and Significant Dates (J. R. Stockton), an extensive list of dates that are problematic for various operating systems and computing devices
- Dates potentially causing problems in computer systems