epoch

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See also: Geologic Timescale (table)Geologic Timescale
Era Period Epoch Approximate duration
(millions of years)
Approximate number of years ago
(millions of years)

Cenozoic Quaternary Holocene 10,000 years ago to the present  
Pleistocene 2 .
..... Click the link for more information.

epoch,

unit of geologic time that is a subdivision of a period. The Pleistocene and Holocene epochs, for example, are divisions of the Quaternary period. Epoch is also used to describe a short length of geologic time during a special occurrence, such as the glacial epoch. See geologygeology,
science of the earth's history, composition, and structure, and the associated processes. It draws upon chemistry, biology, physics, astronomy, and mathematics (notably statistics) for support of its formulations.
..... Click the link for more information.
; Geologic TimescaleGeologic Timescale
Era Period Epoch Approximate duration
(millions of years)
Approximate number of years ago
(millions of years)

Cenozoic Quaternary Holocene 10,000 years ago to the present  
Pleistocene 2 .
..... Click the link for more information.
 (table).

epoch

An arbitrary fixed date or instant of time that is used as a reference datum, especially for stellar coordinates and orbital elements. For example, the coordinates right ascension and declination are continuously changing, primarily as a result of the precession of the equinoxes. Coordinates must therefore refer to a particular epoch, which can be the time of an observation, the beginning of the year in which a series of observations of an object was made, or the beginning of a half century. The standard epoch specifies the reference system to which coordinates are referred. Coordinates of star catalogs commonly referred to the mean equator and equinox of the beginning of a Besselian year. Since 1984 the Julian year has been used: the current standard epoch, designated J2000.0, is 2000 Jan. 1.5; it is exactly one Julian century removed from the standard epoch of 1900 Jan 0.5. Epochs for the beginning of a year now differ from the standard epoch by multiples of the Julian year. A standard epoch is usually retained for 50 years.

epoch

[′ep·ək]
(astronomy)
A particular instant for which certain data are valid; for example, star positions in an astronomical catalog, epoch 1950.0.
(geology)
A major subdivision of a period of geologic time.
(physics)

epoch

1. Astronomy a precise date to which information, such as coordinates, relating to a celestial body is referred
2. Geology a unit of geological time within a period during which a series of rocks is formed
3. Physics the displacement of an oscillating or vibrating body at zero time

epoch

(operating system)
(Probably from astronomical timekeeping) A term used originally in Unix documentation for the time and date corresponding to zero in an operating system's clock and timestamp values.

Under most Unix versions the epoch is 1970-01-01 00:00:00 GMT; under VMS, it's 1858-11-17 00:00:00 (the base date of the US Naval Observatory's ephemerides); on a Macintosh, it's 1904-01-01 00:00:00.

System time is measured in seconds or ticks past the epoch. Weird problems may ensue when the clock wraps around (see wrap around), which is not necessarily a rare event; on systems counting 10 ticks per second, a signed 32-bit count of ticks is good only for 0.1 * 2**31-1 seconds, or 6.8 years. The one-tick-per-second clock of Unix is good only until 2038-01-18, assuming at least some software continues to consider it signed and that word lengths don't increase by then. See also wall time.

epoch

(editor)
(Epoch) A version of GNU Emacs for the X Window System from NCSA.

epoch

The starting date from which time is measured as a number of days or minutes or seconds, etc. In computer applications, epochs are used to maintain a time reference as a single number for ease of computation. Otherwise, depending on the granularity of time desired, every point in time would have to be stored with some of or all of the components of the time hierarchy, including year, month, day, hour, minute, second, millisecond, microsecond and nanosecond. Following are the various epochs in use. See also EPOC.

System          Epoch            Measured in

  Unix/Linux      Jan. 1, 1970     Seconds
  Java            Jan. 1, 1970     Milliseconds

  Windows files   Jan. 1, 1601     Ticks (100 ns)
  Windows dates   Jan. 1, 0001     Ticks (100 ns)

  Mac             Jan. 1, 2001     Seconds
  Earlier Mac     Jan. 1, 1904     Seconds

  Excel           Dec. 31, 1899    Days
  DB2             Dec. 31, 1899    Days

  Unununium       Jan. 1, 2000     Microseconds



No USB Drives in 1969
When this USB drive was formatted, the date was reset to the epoch. The 7:00pm tells us it was done on the East Coast, five hours behind the UTC time of January 1, 1970 (see above). This info comes from the Mac that the drive was plugged into.