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.
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.
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.
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.
|epoch date||notable uses||rationale for selection|
|January 1, 0||matlab|
|January 1, 1||Symbian, Microsoft .NET, REXX||This epoch date is known as Rata Die|
|January 1, 1601||FAT32 file system, COBOL|
|December 31, 1840||MUMPS programming language||1841 was roughly the birthyear of the world's oldest living person when the language was designed.|
|November 17, 1858||VMS, United States Naval Observatory, other astronomy-related computations|
|December 30, 1899||Microsoft COM DATE||Technical internal value used by Microsoft Excel; to simplify calculations by falsely assuming 1900 to be a leap year.|
|January 0, 1900||Microsoft Excel, Lotus 1-2-3||While logically January 0, 1900 is equivalent to December 31, 1899, these systems do not allow users to specify the latter date.|
|January 1, 1900||Network Time Protocol, IBM CICS, Mathematica|
|January 1, 1904||Apple Inc.'s Mac OS through version 9, Palm OS, MP4||1904 is the first leap year of the twentieth century.|
|January 1, 1960||S-Plus, SAS|
|January 1, 1968||Pick OS|
|January 1, 1978||AmigaOS|
|January 1, 1980||MS DOS, OS/2, FAT16 filesystem|
|January 6, 1980||Qualcomm BREW, GPS|
|January 1, 2000||Various updated MS DOS legacy software||Retrofitting to address the Year 2000 problem|
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 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.
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.