Epoch (reference date)

In the fields of chronology and periodization, an epoch means an instant in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured. Time measurement units are counted from the epoch so that the date and time of events can be specified unambiguously.

Events taking place before the epoch can be dated by counting negatively from the epoch, though in pragmatic periodization practice, epochs are defined for the past, and another epoch is used to start the next era, therefore serving as the ending of the older preceding era. The whole purpose and criteria of such definitions is to clarify and co-ordinate scholarship about a period, at times, across disciplines.

Epochs are generally chosen to be convenient or significant by a consensus of the time scale's initial users, or by authoritarian fiat. The epoch moment or date is usually defined by "a specific clear event", condition, or criteria— the epoch event or epoch criteria —from which the period or era or age is usually characterized or described. :Examples:::by events: :::The assassination of the Roman Emperor Alexander Severus triggering the Crisis of the Third Century:::The defenestration of Prague triggering the horrible depopulation of Europe and the Thirty Years' War:::Queen Victoria ascending to the throne giving the start of the Victorian era

::by criteria::::The spurt in exploration, mercantilism, and colonization in the Age of Discovery:::Particular ratios of animal fossils in a rock strata —various Geology epochs

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 anno Domini calendar era (the civil calendar era used internationally and in many countries) is the traditionally-reckoned Incarnation of Jesus. [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.] Many other current and historical calendar eras exist, each with its own epoch.

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.
* A similar system existed in China before 1912, being based on the accession year of the emperor (1911 was thus the fourth year of the Xuantong period). With the establishment of the Republic of China in 1912, the republican era was introduced. It is still very common in Taiwan to date events via the republican era. The People's Republic of China adopted the common era calendar in 1949 (the 38th year of the Chinese Republic).
* 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 "autarky, self-reliance".
* In Thailand in 1888 King Chulalongkorn decreed a National Thai Era dating from the founding of Bangkok on April 6, 1782. In 1912, 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 Thai solar calendar using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar.

Religious eras

* 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. The beginning of year 1 of the Hebrew calendar occurred in the autumn of 3761 BC. Therefore, "Rosh Hashanah, the Jewish New Year, in September 2003 marked the transition from 5763 to 5764". [ [http://www.myjewishlearning.com/holidays/About_Jewish_Holidays/Solar_and_Lunar/CountingYears.htm My Jewish Learning:Counting the Years] ] [ [http://www.rosettacalendar.com/calendars.html Rosetta Calendar] ]
* 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.

Other

*In the French Republican Calendar, a calendar used by the French government for about twelve years from late 1793, the epoch was the beginning of the "Republican Era", September 22, 1792 (the day the French First Republic was proclaimed, one day after the Convention abolished the monarchy).
*In the scientific Before Present system of numbering years for purposes of radiocarbon dating, the reference date is January 1, 1950 (though the use of January 1 is mostly irrelevant, as radiocarbon dating is approximate to years and days can rarely be calculated accurately).
*Different branches of Freemasonry have selected different years to date their documents according to a Masonic era, such has the Anno Lucis (A.L.).

Astronomy

In astronomy, an epoch is a specific moment in time for which celestial coordinates or orbital elements are specified, and from which other orbital parametrics are thereafter calculated in order to predict future position. The applied tools of the mathematics disciplines of Celestial mechanics or its subfield Orbital mechanics (both predict orbital paths and positions) about a center of gravity are used to generate an ephemeris (plural: "ephemerides"; from the Greek word "ephemeros" = daily) which is a table of values that gives the positions of astronomical objects in the sky at a given time or times, or a formula to calculate such given the proper time offset from the epoch. Such calculations generally result in an elliptical path on a plane defined by some point on the orbit, and the two focii of the ellipse. Viewing from another orbiting body, following its own trace and orbit, creates shifts in three dimensions in the spherical trigonometry used to calculate relative positions. Interestingly, these dynamics in three dimensions are also elliptical, which means the ephemeris need only specify one set of equations to be a useful predictive tool to predict future location of the object of interest.

Over time, inexactitudes and other errors accumulate, creating more and greater errors of prediction, so ephemeris factors need recalculated from time to time, and that requires a new epoch to be defined. Different astronomers or groups of astronomers used to define epochs to suit themselves, but these days of speedy communications, the epochs are generally defined in an international agreement, so astronomers world wide can collaborate more effectively. It was inefficient and error prone for data observed by one group to need translation (mathematic transformation) so other groups could compare information.

J2000.0

The current standard epoch is called "J2000.0" (Julian 2000, based on the Julian Calendar), is defined by international agreement, and is precisely defined to be approximately noon at the Royal Observatory, Greenwich, in London England:
# The Julian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT.
# This is equivalent to January 1, 2000, 11:59:27.816 TAI (International Atomic Time) or
# January 1, 2000, 11:58:55.816 UTC (Coordinated Universal Time).

Computing

The time kept internally by a computer system is usually expressed as the number of time units that have elapsed since a specified epoch, which is nearly always specified as midnight Universal Time on some particular date.

Software timekeeping systems vary widely in the granularity of their time units; some systems may use time units as large as a day, while others may use nanoseconds. For example, for an epoch date of midnight UTC on January 1, 1900, and a time unit of a second, the time of midnight UTC on January 2, 1900 is represented by the number 86400, the number of seconds in one day.

These representations of time are mainly for internal use. If an end user interaction with dates and times is required, the software will nearly always convert this internal number into a date and time representation that is comprehensible to humans.

Notable epoch dates in computing

The following table lists epoch dates used by popular software and other computer-related systems. The time in these systems is stored as the number some time unit (days, seconds, nanoseconds, etc) that have elapsed since midnight UTC on the given date.

Problems with epoch-based computer time representation

Computers are in general unable to store arbitrarily large numbers. Instead, each number stored by a computer is allotted a fixed amount of space. Therefore, when the number of time units that have elapsed since a system's epoch exceeds the largest number that can fit in the space allotted to the time representation, the time representation overflows, and problems can occur. While a system's behavior after overflow occurs is not necessarily predictable, one likely result is that the number representing the time will reset to zero, and the computer system will think that the current time is the epoch time again.

Most famously, older systems which counted time as the number of years elapsed since the epoch of January 1, 1900 and which only allotted enough space to store the numbers 0 through 99, experienced the Year 2000 problem. These systems (if not corrected beforehand) would interpret the date January 1, 2000 as January 1, 1900, leading to unpredictable errors at the beginning of the year 2000.

Even systems which allot much more storage to the time representation are not immune from this kind of error. Many UNIX-like operating systems which keep time as seconds elapsed from the epoch date of January 1, 1970, and allot timekeeping enough storage to store numbers as large as nowrap|2 147 483 647 will experience an overflow problem on January 19, 2038 if not fixed beforehand. This is known as the Year 2038 problem. A correction involving doubling the storage allocated to timekeeping on these systems will allow them to represent dates more than 290 billion years into the future.

Other more subtle timekeeping problems exist in computing, such as accounting for leap seconds, which are not observed with any predictability or regularity. Additionally, applications which needs to represent historical dates and times (for example, representing a date prior to the switch from the Julian calendar to the Gregorian calendar) must use specialized timekeeping libraries.

Finally, some software must maintain compatibility with older software that does not keep time in strictly accordance with traditional timekeeping systems. For example, Microsoft Excel observes the fictional date of February 29, 1900 in order to maintain compatibility with older versions of Lotus 1-2-3, which observed that date to expidite time calculations at a time when computers were much slower.

Geology

In geology, an Epoch is a time division criteria based on specific physical and chemical characteristics in rock layers (Lithography) indicative of the global environment, including tectonic activity (mountain building, continental drift, etc.) and include macroscopic (visible by inspection) and microscopic Fauna features or characteristics that are clear and distinct from other series defining criteria. For example, the formations, rock beds, and members of differing rock types that were being laid down in different environments at the same time.

Each defined set of characteristics—in a word, Epochs —directly correspond to specific rock series, the equivalent of an "era" (both are measured usually in time units) in rock layers. Since such layers correspond to time (or era) of formation, they are used to date pre-historic events in other sciences.

Notes

External links

* [http://www.merlyn.demon.co.uk/critdate.htm Critical and Significant Dates] (J. R. Stockton), an extensive list of dates that are problematic for various operating systems and computing devices
* [http://www.theiet.org/factfiles/it/dates.cfm?type=pdf Potential problem dates for computers] A list of potential problem dates for computers and software from 2001 to 2100.