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Unix time

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"January 1, 1970" redirects here. Not to be confused withJanuary 1, 1970 (date).
"Epoch time" redirects here. For other epochs, seeEpoch (computing).For the newspaper, seeThe Epoch Times.

Current Unix time
1720408749(refresh page to update)
2024-07-08T03:19:09+00:00
Unix time passed1000000000seconds on 2001-09-09T01:46:40Z.[1]It was celebrated in Copenhagen, Denmark at a party held by theDanish UNIX User Groupat 03:46:40 local time.

Unix time[a]is a date and time representation widely used incomputing.It measures time by the number of non-leap secondsthat have elapsed since 00:00:00UTCon 1 January 1970, the Unixepoch.In modern computing, values are sometimes stored with highergranularity,such asmicrosecondsornanoseconds.

Unix time originated as thesystem timeofUnixoperating systems. It has come to be widely used in other computeroperating systems,file systems,programming languages,anddatabases.

Definition[edit]

Unix time is currently defined as the number of non-leap seconds which have passed since 00:00:00UTC on Thursday, 1 January 1970, which is referred to as theUnixepoch.[3]Unix time is typically encoded as asigned integer.

The Unix time0is exactly midnight UTC on 1 January 1970, with Unix time incrementing by 1 for every non-leap second after this. For example, 00:00:00UTC on 1 January 1971 is represented in Unix time as31536000.Negative values, on systems that support them, indicate times before the Unix epoch, with the value decreasing by 1 for every non-leap second before the epoch. For example, 00:00:00UTC on 1 January 1969 is represented in Unix time as−31536000.Every day in Unix time consists of exactly86400seconds.

Unix time is sometimes referred to asEpoch time.This can be misleading since Unix time is not the only time system based on an epoch and the Unix epoch is not the only epoch used by other time systems.[5]

Leap seconds[edit]

Unix time differs from bothCoordinated Universal Time(UTC) andInternational Atomic Time(TAI) in its handling ofleap seconds.UTC includes leap seconds that adjust for the discrepancy between precise time, as measured byatomic clocks,andsolar time,relating to the position of the earth in relation to the sun.International Atomic Time(TAI), in which every day is precisely86400seconds long, ignores solar time and gradually losessynchronizationwith the Earth's rotation at a rate of roughly one second per year. In Unix time, every day contains exactly86400seconds. Each leap second uses thetimestampof a second that immediately precedes or follows it.[3]

On a normal UTC day, which has a duration of86400seconds, the Unix time number changes in acontinuousmanner across midnight. For example, at the end of the day used in the examples above, the time representations progress as follows:

Unix time across midnight into 17 September 2004 (no leap second)
TAI (17 September 2004) UTC (16 to 17 September 2004) Unix time
2004-09-17T00:00:30.75 2004-09-16T23:59:58.75 1095379198.75
2004-09-17T00:00:31.00 2004-09-16T23:59:59.00 1095379199.00
2004-09-17T00:00:31.25 2004-09-16T23:59:59.25 1095379199.25
2004-09-17T00:00:31.50 2004-09-16T23:59:59.50 1095379199.50
2004-09-17T00:00:31.75 2004-09-16T23:59:59.75 1095379199.75
2004-09-17T00:00:32.00 2004-09-17T00:00:00.00 1095379200.00
2004-09-17T00:00:32.25 2004-09-17T00:00:00.25 1095379200.25
2004-09-17T00:00:32.50 2004-09-17T00:00:00.50 1095379200.50
2004-09-17T00:00:32.75 2004-09-17T00:00:00.75 1095379200.75
2004-09-17T00:00:33.00 2004-09-17T00:00:01.00 1095379201.00
2004-09-17T00:00:33.25 2004-09-17T00:00:01.25 1095379201.25

When aleap secondoccurs, the UTC day is not exactly86400seconds long and the Unix time number (which always increases by exactly86400each day) experiences adiscontinuity.Leap seconds may be positive or negative. No negative leap second has ever been declared, but if one were to be, then at the end of a day with a negative leap second, the Unix time number would jump up by 1 to the start of the next day. During a positive leap second at the end of a day, which occurs about every year and a half on average, the Unix time number increases continuously into the next day during the leap second and then at the end of the leap second jumps back by 1 (returning to the start of the next day). For example, this is what happened on strictly conformingPOSIX.1systems at the end of 1998:

Unix time across midnight into 1 January 1999 (positive leap second)
TAI (1 January 1999) UTC (31 December 1998 to 1 January 1999) Unix time
1999-01-01T00:00:29.75 1998-12-31T23:59:58.75 915148798.75
1999-01-01T00:00:30.00 1998-12-31T23:59:59.00 915148799.00
1999-01-01T00:00:30.25 1998-12-31T23:59:59.25 915148799.25
1999-01-01T00:00:30.50 1998-12-31T23:59:59.50 915148799.50
1999-01-01T00:00:30.75 1998-12-31T23:59:59.75 915148799.75
1999-01-01T00:00:31.00 1998-12-31T23:59:60.00 915148800.00
1999-01-01T00:00:31.25 1998-12-31T23:59:60.25 915148800.25
1999-01-01T00:00:31.50 1998-12-31T23:59:60.50 915148800.50
1999-01-01T00:00:31.75 1998-12-31T23:59:60.75 915148800.75
1999-01-01T00:00:32.00 1999-01-01T00:00:00.00 915148800.00
1999-01-01T00:00:32.25 1999-01-01T00:00:00.25 915148800.25
1999-01-01T00:00:32.50 1999-01-01T00:00:00.50 915148800.50
1999-01-01T00:00:32.75 1999-01-01T00:00:00.75 915148800.75
1999-01-01T00:00:33.00 1999-01-01T00:00:01.00 915148801.00
1999-01-01T00:00:33.25 1999-01-01T00:00:01.25 915148801.25

Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number1483142400is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00). In the theoretical case when a negative leap second occurs, no ambiguity is caused, but instead there is a range of Unix time numbers that do not refer to any point in UTC time at all.

A Unix clock is often implemented with a different type of positive leap second handling associated with theNetwork Time Protocol(NTP). This yields a system that does not conform to the POSIX standard. See the section below concerning NTP for details.

When dealing with periods that do not encompass a UTC leap second, the difference between two Unix time numbers is equal to the duration in seconds of the period between the corresponding points in time. This is a common computational technique. However, where leap seconds occur, such calculations give the wrong answer. In applications where this level of accuracy is required, it is necessary to consult a table of leap seconds when dealing with Unix times, and it is often preferable to use a different time encoding that does not suffer from this problem.

A Unix time number is easily converted back into a UTC time by taking the quotient and modulus of the Unix time number, modulo86400.The quotient is the number of days since the epoch, and the modulus is the number of seconds since midnight UTC on that day. If given a Unix time number that is ambiguous due to a positive leap second, this algorithm interprets it as the time just after midnight. It never generates a time that is during a leap second. If given a Unix time number that is invalid due to a negative leap second, it generates an equally invalid UTC time. If these conditions are significant, it is necessary to consult a table of leap seconds to detect them.

Non-synchronous Network Time Protocol-based variant[edit]

Commonly aMills-style Unix clock is implemented with leap second handling not synchronous with the change of the Unix time number. The time number initially decreases where a leap should have occurred, and then it leaps to the correct time 1 second after the leap. This makes implementation easier, and is described by Mills' paper.[6]This is what happens across a positive leap second:

Non-synchronous Mills-style Unix clock
across midnight into 1 January 1999 (positive leap second)
TAI (1 January 1999) UTC (31 December 1998 to 1 January 1999) State Unix clock
1999-01-01T00:00:29.75 1998-12-31T23:59:58.75 TIME_INS 915148798.75
1999-01-01T00:00:30.00 1998-12-31T23:59:59.00 TIME_INS 915148799.00
1999-01-01T00:00:30.25 1998-12-31T23:59:59.25 TIME_INS 915148799.25
1999-01-01T00:00:30.50 1998-12-31T23:59:59.50 TIME_INS 915148799.50
1999-01-01T00:00:30.75 1998-12-31T23:59:59.75 TIME_INS 915148799.75
1999-01-01T00:00:31.00 1998-12-31T23:59:60.00 TIME_INS 915148800.00
1999-01-01T00:00:31.25 1998-12-31T23:59:60.25 TIME_OOP 915148799.25
1999-01-01T00:00:31.50 1998-12-31T23:59:60.50 TIME_OOP 915148799.50
1999-01-01T00:00:31.75 1998-12-31T23:59:60.75 TIME_OOP 915148799.75
1999-01-01T00:00:32.00 1999-01-01T00:00:00.00 TIME_OOP 915148800.00
1999-01-01T00:00:32.25 1999-01-01T00:00:00.25 TIME_WAIT 915148800.25
1999-01-01T00:00:32.50 1999-01-01T00:00:00.50 TIME_WAIT 915148800.50
1999-01-01T00:00:32.75 1999-01-01T00:00:00.75 TIME_WAIT 915148800.75
1999-01-01T00:00:33.00 1999-01-01T00:00:01.00 TIME_WAIT 915148801.00
1999-01-01T00:00:33.25 1999-01-01T00:00:01.25 TIME_WAIT 915148801.25

This can be decoded properly by paying attention to the leap second state variable, which unambiguously indicates whether the leap has been performed yet. The state variable change is synchronous with the leap.

A similar situation arises with a negative leap second, where the second that is skipped is slightly too late. Very briefly the system shows a nominally impossible time number, but this can be detected by the TIME_DEL state and corrected.

In this type of system the Unix time number violates POSIX around both types of leap second. Collecting the leap second state variable along with the time number allows for unambiguous decoding, so the correct POSIX time number can be generated if desired, or the full UTC time can be stored in a more suitable format.

The decoding logic required to cope with this style of Unix clock would also correctly decode a hypothetical POSIX-conforming clock using the same interface. This would be achieved by indicating the TIME_INS state during the entirety of an inserted leap second, then indicating TIME_WAIT during the entirety of the following second while repeating the seconds count. This requires synchronous leap second handling. This is probably the best way to express UTC time in Unix clock form, via a Unix interface, when the underlying clock is fundamentally untroubled by leap seconds.

Variant that counts leap seconds[edit]

Another, much rarer, non-conforming variant of Unix time keeping involves incrementing the value for all seconds, including leap seconds;[7]some Linux systems are configured this way.[8]Time kept in this fashion is sometimes referred to as "TAI" (although timestamps can be converted to UTC if the value corresponds to a time when the difference between TAI and UTC is known), as opposed to "UTC" (although not all UTC time values have a unique reference in systems that do not count leap seconds).[8]

Because TAI has no leap seconds, and every TAI day is exactly 86400 seconds long, this encoding is actually a pure linear count of seconds elapsed since 1970-01-01T00:00:10TAI. This makes time interval arithmetic much easier. Time values from these systems do not suffer the ambiguity that strictly conforming POSIX systems or NTP-driven systems have.

In these systems it is necessary to consult a table of leap seconds to correctly convert between UTC and the pseudo-Unix-time representation. This resembles the manner in which time zone tables must be consulted to convert to and fromcivil time;theIANA time zone databaseincludes leap second information, and the sample code available from the same source uses that information to convert between TAI-based timestamps and local time. Conversion also runs into definitional problems prior to the 1972 commencement of the current form of UTC (see sectionUTC basisbelow).

This system, despite its superficial resemblance, is not Unix time. It encodes times with values that differ by several seconds from the POSIX time values. A version of this system, in which the epoch was 1970-01-01T00:00:00TAI rather than 1970-01-01T00:00:10TAI, was proposed for inclusion in ISO C'stime.h,but only the UTC part was accepted in 2011.[9]Atai_clockdoes, however, exist in C++20.

Representing the number[edit]

A Unix time number can be represented in any form capable of representing numbers. In some applications the number is simply represented textually as a string of decimal digits, raising only trivial additional problems. However, certain binary representations of Unix times are particularly significant.

The Unixtime_tdata type that represents a point in time is, on many platforms, asigned integer,traditionally of 32bits(but see below), directly encoding the Unix time number as described in the preceding section. A signed 32-bit value covers about 68 years before and after the 1970-01-01 epoch. The minimum representable date is Friday 1901-12-13, and the maximum representable date is Tuesday 2038-01-19. One second after 03:14:07UTC 2038-01-19 this representation willoverflowin what is known as theyear 2038 problem.

In some newer operating systems,time_thas been widened to 64 bits. This expands the times representable to about292.3 billion yearsin both directions, which is over twenty times the presentage of the universe.

There was originally some controversy over whether the Unixtime_tshould be signed or unsigned. If unsigned, its range in the future would be doubled, postponing the 32-bit overflow (by 68 years). However, it would then be incapable of representing times prior to the epoch. The consensus is fortime_tto be signed, and this is the usual practice. The software development platform for version 6 of theQNXoperating system has an unsigned 32-bittime_t,though older releases used a signed type.

ThePOSIXandOpen GroupUnix specifications include theC standard library,which includes the time types and functions defined in the<time.h>header file. The ISO C standard states thattime_tmust be an arithmetic type, but does not mandate any specific type or encoding for it. POSIX requirestime_tto be an integer type, but does not mandate that it be signed or unsigned.

Unix has no tradition of directly representing non-integer Unix time numbers as binary fractions. Instead, times with sub-second precision are represented usingcomposite data typesthat consist of two integers, the first being atime_t(the integral part of the Unix time), and the second being the fractional part of the time number in millionths (instruct timeval) or billionths (instruct timespec).[10][11]These structures provide adecimal-basedfixed-pointdata format, which is useful for some applications, and trivial to convert for others.

UTC basis[edit]

The present form of UTC, with leap seconds, is defined only starting from 1 January 1972. Prior to that, since 1 January 1961 there was an older form of UTC in which not only were there occasional time steps, which were by non-integer numbers of seconds, but also the UTC second was slightly longer than the SI second, and periodically changed to continuously approximate the Earth's rotation. Prior to 1961 there was no UTC, and prior to 1958 there was no widespreadatomic timekeeping;in these eras, some approximation ofGMT(based directly on the Earth's rotation) was used instead of an atomic timescale.[citation needed]

The precise definition of Unix time as an encoding of UTC is only uncontroversial when applied to the present form of UTC. The Unix epoch predating the start of this form of UTC does not affect its use in this era: the number of days from 1 January 1970 (the Unix epoch) to 1 January 1972 (the start of UTC) is not in question, and the number of days is all that is significant to Unix time.

The meaning of Unix time values below+63072000(i.e., prior to 1 January 1972) is not precisely defined. The basis of such Unix times is best understood to be an unspecified approximation of UTC. Computers of that era rarely had clocks set sufficiently accurately to provide meaningful sub-second timestamps in any case. Unix time is not a suitable way to represent times prior to 1972 in applications requiring sub-second precision; such applications must, at least, define which form of UT or GMT they use.

As of 2009,the possibility of ending the use of leap seconds in civil time is being considered.[12]A likely means to execute this change is to define a new time scale, calledInternational Time[citation needed],that initially matches UTC but thereafter has no leap seconds, thus remaining at a constant offset from TAI. If this happens, it is likely that Unix time will be prospectively defined in terms of this new time scale, instead of UTC. Uncertainty about whether this will occur makes prospective Unix time no less predictable than it already is: if UTC were simply to have no further leap seconds the result would be the same.

History[edit]

The earliest versions of Unix time had a 32-bit integer incrementing at a rate of 60Hz,which was the rate of the system clock on the hardware of the early Unix systems. Timestamps stored this way could only represent a range of a little over two and a quarter years. The epoch being counted from was changed with Unix releases to prevent overflow, with midnight on 1 January 1971 and 1 January 1972 both being used as epochs during Unix's early development. Early definitions of Unix time also lacked timezones.[13][14]

The current epoch of 1 January 1970 00:00:00 UTC was selected arbitrarily by Unix engineers because it was considered a convenient date to work with. The precision was changed to count in seconds in order to avoid short-term overflow.[1]

WhenPOSIX.1was written, the question arose of how to precisely definetime_tin the face of leap seconds. The POSIX committee considered whether Unix time should remain, as intended, a linear count of seconds since the epoch, at the expense of complexity in conversions with civil time or a representation of civil time, at the expense of inconsistency around leap seconds. Computer clocks of the era were not sufficiently precisely set to form a precedent one way or the other.

The POSIX committee was swayed by arguments against complexity in the library functions,[citation needed]and firmly defined the Unix time in a simple manner in terms of the elements of UTC time. This definition was so simple that it did not even encompass the entireleap yearrule of the Gregorian calendar, and would make 2100 a leap year.

The 2001 edition of POSIX.1 rectified the faulty leap year rule in the definition of Unix time, but retained the essential definition of Unix time as an encoding of UTC rather than a linear time scale. Since the mid-1990s, computer clocks have been routinely set with sufficient precision for this to matter, and they have most commonly been set using the UTC-based definition of Unix time. This has resulted in considerable complexity in Unix implementations, and in theNetwork Time Protocol,to execute steps in the Unix time number whenever leap seconds occur.[citation needed]

Usage[edit]

Unix time is widely adopted in computing beyond its original application as the system time forUnix.Unix time is available in almost all system programmingAPIs,including those provided by both Unix-based and non-Unixoperating systems.Almost all modernprogramming languagesprovide APIs for working with Unix time or converting them to another data structure. Unix time is also used as a mechanism for storing timestamps in a number offile systems,file formats,anddatabases.

TheC standard libraryuses Unix time for all date and time functions, and Unix time is sometimes referred to as time_t, the name of thedata typeused for timestamps inCandC++.C's Unix time functions are defined as the system time API in thePOSIXspecification.[15]The C standard library is used extensively in all modern desktop operating systems, includingMicrosoft WindowsandUnix-likesystems such asmacOSandLinux,where it is a standard programming interface.[16][17][18]

iOSprovides a Swift API which defaults to using an epoch of 1 January 2001 but can also be used with Unix timestamps.[19]Androiduses Unix time alongside a timezone for its system time API.[20]

Windows does not use Unix time for storing time internally but does use it in system APIs, which are provided in C++ and implement the C standard library specification.[16]Unix time is used in thePE formatfor Windows executables.[21]

Unix time is typically available in major programming languages and is widely used in desktop, mobile, and web application programming.Javaprovides an Instant object which holds a Unix timestamp in both seconds and nanoseconds.[22]Pythonprovides a time library which uses Unix time.[23]JavaScriptprovides a Date library which provides and stores timestamps in milliseconds since the Unix epoch and is implemented in all modern desktop and mobileweb browsersas well as in JavaScript server environments likeNode.js.[24]

Filesystems designed for use with Unix-based operating systems tend to use Unix time.APFS,the file system used by default across all Apple devices, andext4,which is widely used on Linux and Android devices, both use Unix time in nanoseconds for file timestamps.[25][26]Severalarchive file formatscan store timestamps in Unix time, includingRARandtar.[27][28]Unix time is also commonly used to store timestamps in databases, including inMySQLandPostgreSQL.[29][30]

Limitations[edit]

Unix time was designed to encode calendar dates and times in a compact manner intended for use by computers internally. It is not intended to be easily read by humans or to store timezone-dependent values. It is also limited by default to representing time in seconds, making it unsuited for use when a more precise measurement of time is needed, such as when measuring the execution time of programs.[31]

Range of representable times[edit]

See also:Year 2038 problem
An animated visual of the32-bit Unix time overflow which will occur in 2038

Unix time by design does not require a specific size for the storage, but most common implementations of Unix time use asigned integerwith theword sizeof the underlying computer. As the majority of modern computers are32-bitor64-bit,and a large number of programs are still written in 32-bit compatibility mode, this means that many programs using Unix time are using signed 32-bit integer fields. The maximum value of a signed 32 bit integer is 231-1, and the minimum value is -(231), making it impossible to represent dates before 13 December 1901 (at 20:45:52 UTC) or after 19 January 2038 (at 03:14:07 UTC). The early cutoff can have an impact on databases that are storing historical information; in some databases where 32-bit Unix time is used for timestamps, it may be necessary to store time in a different form of field, such as a string, to represent dates before 1901. The late cutoff is known as theYear 2038 problemand has the potential to cause issues as the date approaches, as dates beyond the 2038 cutoff would wrap back around to the start of the representable range in 1901.[31]: 60 

Date range cutoffs are not an issue with 64-bit representations of Unix time, as the effective range of dates representable with Unix time stored in a signed 64-bit integer is over 584 billion years, or 292 billion years in either direction of the 1970 epoch.[31]: 60-61 [32]

Alternatives[edit]

Unix time is not the only standard for time that counts away from an epoch. OnWindows,theFILETIMEtype stores time as a count of 100-nanosecond intervals that have elapsed since 0:00GMTon 1 January 1601.[33]Windows epoch time is used to store timestamps for files[34]and in protocols such as theActive DirectoryTime Service[35]andServer Message Block.

TheNetwork Time Protocolused to coordinate time between computers uses an epoch of 1 January 1900, counted in an unsigned 32-bit integer for seconds and another unsigned 32-bit integer for fractional seconds, which rolls over every 232seconds (about once every 136 years).[36]

Many applications and programming languages provide methods for storing time with an explicit timezone.[37]There are also a number of time format standards which exist to be readable by both humans and computers, such asISO 8601.

Notable events in Unix time[edit]

Unix enthusiasts have a history of holding "time_t parties" (pronounced "timetea parties") to celebrate significant values of the Unix time number.[38][39]These are directly analogous to thenew yearcelebrations that occur at the change of year in many calendars. As the use of Unix time has spread, so has the practice of celebrating its milestones. Usually it is time values that are round numbers indecimalthat are celebrated, following the Unix convention of viewingtime_tvalues in decimal. Among some groups roundbinarynumbers are also celebrated, such as +230which occurred at 13:37:04 UTC on Saturday, 10 January 2004.[citation needed]

The events that these celebrate are typically described as "Nseconds since the Unix epoch ", but this is inaccurate; as discussed above, due to the handling of leap seconds in Unix time the number of seconds elapsed since the Unix epoch is slightly greater than the Unix time number for times later than the epoch.

  • At 18:36:57 UTC on Wednesday, 17 October 1973, the first appearance of the date inISO 8601format[b](1973-10-17)within the digits of Unix time (119731017) took place.
  • At 01:46:40 UTC on Sunday, 9 September 2001, the Unix billennium (Unix time number1000000000) was celebrated.[40]The namebillenniumis aportmanteauofbillionandmillennium.[41][42]Some programs which stored timestamps using a text representation encountered sorting errors, as in a text sort, times after the turnover starting with a1digit erroneously sorted before earlier times starting with a9digit. Affected programs included the popularUsenetreaderKNodeande-mailclientKMail,part of theKDEdesktop environment. Such bugs were generally cosmetic in nature and quickly fixed once problems became apparent.[citation needed]The problem also affected manyFiltrixdocument-format filters provided withLinuxversions ofWordPerfect;a patch was created by the user community to solve this problem, sinceCorelno longer sold or supported that version of the program.[43]
  • At 23:31:30 UTC on Friday, 13 February 2009, thedecimalrepresentation of Unix time reached1234567890seconds.[44]Googlecelebrated this with aGoogle doodle.[45]Parties and other celebrations were held around the world, among various technical subcultures, to celebrate the1234567890th second.[38][46]

In popular culture[edit]

Vernor Vinge's novelA Deepness in the Skydescribes a spacefaring trading civilization thousands of years in the future that still uses the Unix epoch. The "programmer-archaeologist"responsible for finding and maintaining usable code in mature computer systems first believes that the epoch refers to the time whenman first walked on the Moon,but then realizes that it is "the 0-second of one of humankind's first computer operating systems".[47]

See also[edit]

Notes[edit]

  1. ^Unix time is also known as "Epoch time", "POSIXtime ",[2]"seconds since the Epoch",[3]"Unix timestamp" or "UNIX Epoch time".[4]
  2. ^cited retroactively since ISO 8601 was published in 1988.

References[edit]

  1. ^abFarhad, Manjoo (8 September 2001)."Unix Tick Tocks to a Billion".Wired.ISSN1059-1028.Archivedfrom the original on 11 September 2022.Retrieved16 October2022.
  2. ^"The Open Group Base Specifications Issue 7, Rationale: Base Definitions, section A.4 General Concepts".The Open Group.Archivedfrom the original on 15 November 2017.Retrieved9 September2019.
  3. ^abc"The Open Group Base Specifications Issue 7, section 4.16 Seconds Since the Epoch".The Open Group.Archivedfrom the original on 22 December 2017.Retrieved22 January2017.
  4. ^Matthew, Neil; Stones, Richard (2008). "The Linux Environment".Beginning Linux Programming.Indianapolis, Indiana, US: Wiley. p. 148.ISBN978-0-470-14762-7.
  5. ^"FILETIME structure (minwinbase.h)".Microsoft Docs.2 April 2021.
  6. ^Mills, David L. (12 May 2012)."The NTP Timescale and Leap Seconds".eecis.udel.edu.Archivedfrom the original on 15 May 2012.Retrieved21 August2017.
  7. ^"Precision timekeeping".Sources for time zone and daylight saving time data.Archivedfrom the original on 16 October 2017.Retrieved30 May2022.The tz code and data support leap seconds via an optional "right" configuration where a computer's internal time_t integer clock counts every TAI second, as opposed to the default "posix" configuration where the internal clock ignores leap seconds. The two configurations agree for timestamps starting with 1972-01-01 00:00:00 UTC (time_t 63 072 000) and diverge for timestamps starting with time_t 78 796 800, which corresponds to the first leap second 1972-06-30 23:59:60 UTC in the "right" configuration, and to 1972-07-01 00:00:00 UTC in the "posix" configuration.
  8. ^ab"Time Scales".Network Time Protocol Wiki.24 July 2019.Archivedfrom the original on 12 January 2020.Retrieved12 January2020.
  9. ^Markus Kuhn."Modernized API for ISO C".cl.cam.ac.uk.Archivedfrom the original on 26 September 2020.Retrieved31 August2020.
  10. ^"timespec".NetBSD Manual Pages.12 April 2011.Archivedfrom the original on 10 August 2019.Retrieved5 July2019.
  11. ^"time.h(0P)".Linux manual page.Archivedfrom the original on 27 June 2019.Retrieved5 July2019.
  12. ^McCarthy, D. D.;Seidelmann, P. K. (2009).TIME—From Earth Rotation to Atomic Physics.Weinheim: Wiley–VCH Verlag GmbH & Co. KGaA. p. 232.ISBN978-3-527-40780-4.
  13. ^Unix Programmer's Manual(PDF)(1st ed.). 3 November 1971.Archived(PDF)from the original on 5 March 2022.Retrieved28 March2012.timereturns the time since 00:00:00, Jan. 1, 1971, measured in sixtieths of a second.
  14. ^Unix Programmer's Manual(PDF)(3rd ed.). 15 March 1972.Archived(PDF)from the original on 12 February 2023.Retrieved11 February2023.timereturns the time since 00:00:00, Jan. 1, 1972, measured in sixtieths of a second...The time is stored in 32 bits. This guarantees a crisis every 2.26 years.
  15. ^"The Open Group Technical Standard Base Specifications Issue 7 (2018 edition)".IEEE and The Open Group.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.
  16. ^ab"time, _time32, _time64".learn.microsoft.net.Microsoft Corporation. 13 February 2023.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.
  17. ^"The GNU C Library (glibc)".The GNU Operating Sisyem.Free Software Foundation.Archivedfrom the original on 22 April 2016.Retrieved1 May2023.The GNU C Library project provides the core libraries for the GNU system and GNU/Linux systems, as well as many other systems that use Linux as the kernel.
  18. ^"Mac OS X Manual Page for localtime(3)".Apple Documentation Archive.Apple Inc.Archivedfrom the original on 22 July 2022.Retrieved1 May2023.
  19. ^"NSDate".Apple Developer Documentation.Apple Inc.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.
  20. ^"Time Overview".Android Open Source Project.Google LLC.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.
  21. ^"PE Format - Win32 apps".learn.microsoft.Microsoft Corporation. 24 March 2023.Archivedfrom the original on 29 April 2023.Retrieved1 May2023.
  22. ^"Instant (Java Platform SE 8 )".docs.oracle.Oracle.Archivedfrom the original on 25 November 2016.Retrieved1 May2023.
  23. ^"time — Time access and conversions",Python documentation,archivedfrom the original on 22 July 2022,retrieved25 July2022
  24. ^"Date - JavaScript | MDN".developer.mozilla.org.Mozilla.Archivedfrom the original on 21 July 2021.Retrieved1 May2023.
  25. ^Apple File System Reference(PDF),p. 57,archived(PDF)from the original on 5 November 2022,retrieved19 October2022,This timestamp is represented as the number of nanoseconds since January 1, 1970 at 0:00 UTC, disregarding leap seconds
  26. ^"Data Structures and Algorithms".The Linux Kernel documentation.Linux Kernel Organization, Inc.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.
  27. ^"RAR 5.0 archive format".rarlab.win.rar GmbH.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.Time is stored in Unix time_t format if this flags [sic] is set and in Windows FILETIME format otherwise
  28. ^"Tape Archive (tar) File Format Family".loc.gov.Library of Congress. 7 January 2021.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.
  29. ^"Date and Time Functions",MySQL 8.0 Reference Manual,archivedfrom the original on 19 October 2022,retrieved19 October2022
  30. ^"8.5. Date/Time Types".PostgreSQL Documentation.The PostgreSQL Global Development Group. 9 February 2023.Archivedfrom the original on 1 May 2023.Retrieved1 May2023.
  31. ^abcRochkind, Mark (2004).Advanced UNIX Programing(2nd ed.). Addison-Wesley. pp. 56–63.ISBN978-0-13-141154-8.
  32. ^Saxena, Ashutosh; Rawat, Sanjay."IDRBT Working Paper No. 9"(PDF).Archived fromthe original(PDF)on 13 May 2012.
  33. ^"FILETIME (minwinbase.h) - Win32 apps".Microsoft Learn.Microsoft. 2 April 2021.Archivedfrom the original on 10 March 2023.Retrieved9 March2023.
  34. ^"File Times - Win32 apps".Microsoft Learn.Microsoft. 7 January 2021.Archivedfrom the original on 8 March 2023.Retrieved9 March2023.
  35. ^"How to convert date/time attributes in Active Directory to standard time format".Microsoft Learn.Microsoft.Archivedfrom the original on 20 October 2022.Retrieved20 October2022.
  36. ^W. Richard Stevens; Bill Fenner; Andrew M. Rudoff (2004).UNIX Network Programming.Addison-Wesley Professional. pp. 582–.ISBN978-0-13-141155-5.Archivedfrom the original on 30 March 2019.Retrieved16 October2016.
  37. ^"datetime — Basic date and time types".Python Standard Library Reference.Python Software Foundation.Archivedfrom the original on 19 October 2022.Retrieved20 October2022.Attributes: year, month, day, hour, minute, second, microsecond, and tzinfo.
  38. ^abTweney, Dylan (12 February 2009)."Unix Lovers to Party Like It's 1234567890".Wired.Archivedfrom the original on 29 March 2014.Retrieved12 March2017.
  39. ^"Slashdot | date +%s Turning 1111111111".17 March 2005.Archivedfrom the original on 12 January 2020.Retrieved12 January2020.[unreliable source?]
  40. ^"Unix time facts & trivia – Unix Time. Info".Archived fromthe originalon 27 October 2017.
  41. ^"UNIX Approaches Ripe Old Age of One Billion".Electromagnetic.net. Archived fromthe originalon 13 April 2013.Retrieved6 December2012.
  42. ^Neumann, Peter G. (15 October 2001)."The Risks Digest Volume 21: Issue 69".The Risks Digest, Volume 21 Issue 69.21(69). Catless.ncl.ac.uk.Archivedfrom the original on 22 October 2015.Retrieved6 December2012.
  43. ^"Technical Problems".linuxmafia.Archivedfrom the original on 11 October 2012.Retrieved21 August2017.
  44. ^nixCraft."Humor: On Feb, Friday 13, 2009 Unix time Will Be 1234567890".Cyberciti.biz.Retrieved5 July2023.
  45. ^"Google 1234567890 Logo".Google Inc.Archivedfrom the original on 11 January 2013.Retrieved28 January2013.
  46. ^Ahmed, Murad (13 February 2009)."At the third stroke, the Unix time will be 1234567890".The Times.Archivedfrom the original on 14 November 2016.Retrieved12 January2020.
  47. ^Mashey, John R.(27 December 2004)."Languages, Levels, Libraries, and Longevity".Queue.2(9): 32–38.doi:10.1145/1039511.1039532.S2CID28911378.

External links[edit]

Wikiversity has learning resources aboutJavaScript/Epoch time converter
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