System time is measured by a system clock, which is typically implemented as a simple count of the number of ticks that have transpired since some arbitrary starting date, called the epoch. For example, Unix and POSIX-compliant systems encode system time ("Unix time") as the number of seconds elapsed since the start of the Unix epoch at 1 January 1970 00:00:00 UT, with exceptions for leap seconds. Systems that implement the 32-bit and 64-bit versions of the Windows API, such as Windows 9x and Windows NT, provide the system time as both SYSTEMTIME, represented as a year/month/day/hour/minute/second/milliseconds value, and FILETIME, represented as a count of the number of 100-nanosecond ticks since 1 January 1601 00:00:00 UT as reckoned in the proleptic Gregorian calendar.
System time can be converted into calendar time, which is a form more suitable for human comprehension. For example, the Unix system time1000000000 seconds since the beginning of the epoch translates into the calendar time 9 September 2001 01:46:40 UT. Library subroutines that handle such conversions may also deal with adjustments for time zones, daylight saving time (DST), leap seconds, and the user's locale settings. Library routines are also generally provided that convert calendar times into system times.
Closely related to system time is process time, which is a count of the total CPU time consumed by an executing process. It may be split into user and system CPU time, representing the time spent executing user code and system kernel code, respectively. Process times are a tally of CPU instructions or clock cycles and generally have no direct correlation to wall time.
Most first-generation personal computers did not keep track of dates and times. These included systems that ran the CP/M operating system, as well as early models of the Apple II, the BBC Micro, and the Commodore PET, among others. Add-on peripheral boards that included real-time clock chips with on-board battery back-up were available for the IBM PC and XT, but the IBM AT was the first widely available PC that came equipped with date/time hardware built into the motherboard. Prior to the widespread availability of computer networks, most personal computer systems that did track system time did so only with respect to local time and did not make allowances for different time zones.
Microcontrollers operating within embedded systems (such as the Raspberry Pi, Arduino, and other similar systems) do not always have internal hardware to keep track of time. Many such controller systems operate without knowledge of the external time. Those that require such information typically initialize their base time upon rebooting by obtaining the current time from an external source, such as from a time server or external clock, or by prompting the user to manually enter the current time.
Implementation
The system clock is typically implemented as a programmable interval timer that periodically interrupts the CPU, which then starts executing a timer interrupt service routine. This routine typically adds one tick to the system clock (a simple counter) and handles other periodic housekeeping tasks (preemption, etc.) before returning to the task the CPU was executing before the interruption.
Retrieving system time
19 May 2023 08:06:26 UTC
The Wikipedia system time when this page was last generated. → Purge this page, and update this counter.
Note that the resolution of an implementation's measurement of time does not imply the same precision of such measurements. For example, a system might return the current time as a value measured in microseconds, but actually be capable of discerning individual clock ticks with a frequency of only 100 Hz (10 ms).
^ abThe Apple Developer Documentation is not clear on the precision & range of CFAbsoluteTime/CFTimeInterval, except in the CFRunLoopTimerCreate documentation which refers to 'sub-millisecond at most' precision. However, the similar type NSTimeInterval appears to be interchangeable, and has the precision and range listed.
^ abcdThe C standard library does not specify any specific resolution, epoch, range, or datatype for system time values. The C++ library encompasses the C library, so it uses the same system time implementation as C.
^"Time Utilities Reference". Mac OS X Developer Library. 2007.
^ ab"Time Utilities - Foundation". Apple Developer Documentation. Retrieved 6 July 2022.
^Ruth E. Goldenberg; Lawrence J. Kenah; Denise E. Dumas (1991). VAX/VMS Internals and Data Structures, Version 5.2. Digital Press. ISBN978-1555580599.
^"Why is Wednesday, November 17, 1858 the base time for OpenVMS (VAX VMS)?". Stanford University. 24 July 1997. from the original on 24 July 1997. Retrieved 8 January 2020.
^ ab"VSI C Run-Time Library Reference Manual for OpenVMS Systems" (PDF). VSI. November 2020. Retrieved 2021-04-17.
^"OpenVMS and the year 2038". HP. Retrieved 2021-04-17.
^ abz/Architecture Principles of Operation(PDF). Poughkeepsie, New York: International Business Machines. 2007.
^IBM intends to extend the date range on future systems beyond 2042. z/Architecture Principles of Operation, (Poughkeepsie, New York:International Business Machines, 2007) 1-15, 4-45 to 4-47.
^"Expanded 64-bit time values". IBM. Retrieved 2021-04-18.
^Jonathan de Boyne Pollard. "The 32-bit Command Interpreter". On OS/2 Warp 4, date and time can both operate well beyond the year 2000, and even well beyond the year 2038, and in fact up to the year 2079, which is the limit for OS/2 Warp 4's real-time clock.
^ abc"Time and Time Correction in Erlang". www.erlang.org.
^. Microsoft Support. 2003. Archived from the original on 24 October 2007. In the Microsoft Office Spreadsheet Component, the value 0 evaluates to the date December 30, 1899 and the value 1 evaluates to December 31, 1899. ... In Excel, the value 0 evaluates to January 0, 1900 and the value 1 evaluates to January 1, 1900.
^"SYSTEM_CLOCK". Intel Fortran Compiler 19.0 Developer Guide and Reference. 29 April 2019. Retrieved 23 July 2019.
^"SYSTEM_CLOCK — Time function". The GNU Fortran Compiler. Retrieved 27 October 2011.
^ ab"System.nanoTime() method". Java Platform, Standard Edition 6: API Specification. 2015. Retrieved 23 July 2019.
^"Clock.systemUTC() and other methods". Java Platform, Standard Edition 8: API Specification. 2014. Retrieved 15 January 2015.
^"JSR-310 Java Time System". Java Platform, Standard Edition 8: API Specification. 2014. Retrieved 15 January 2015.
^ ab"NSTimeInterval - Foundation". Apple Developer Documentation.
^Douglas Wegscheild, R. Schertler, and Jarkko Hietaniemi, "Time::HiRes". CPAN - Comprehensive Perl Archive Network. 2011. Retrieved 27 October 2011.
^James Britt; Neurogami. "Time class". Ruby-Doc.org: Help and documentation for the Ruby programming language. Scottsdale, AZ. Retrieved 27 October 2011.
^Yugui (18 August 2010). "Ruby 1.9.2 is released". The new 1.9.2 is almost compatible with 1.9.1, except these changes: ... Time is reimplemented. The bug with year 2038 is fixed.
^"Using the Get-Date Cmdlet". Microsoft Docs. Retrieved 23 July 2019.
^"Windows PowerShell Tip of the Week – Formatting Dates and Times". Microsoft Docs. Retrieved 23 July 2019.
External links
Critical and Significant Dates, J. R. Stockton (retrieved 3 December 2015)
The Boost Date/Time Library (C++)
The Boost Chrono Library (C++)
The Chronos Date/Time Library (Smalltalk)
Joda Time, The Joda Date/Time Library (Java)
The Perl DateTime Project 2009-02-19 at the Wayback Machine (Perl)
date: Ruby Standard Library Documentation (Ruby)
May 19, 2023
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System clock redirects here Not to be confused with Clock generator Clock signal or Clock rate In computer science and computer programming system time represents a computer system s notion of the passage of time In this sense time also includes the passing of days on the calendar Unix date command System time is measured by a system clock which is typically implemented as a simple count of the number of ticks that have transpired since some arbitrary starting date called the epoch For example Unix and POSIX compliant systems encode system time Unix time as the number of seconds elapsed since the start of the Unix epoch at 1 January 1970 00 00 00 UT with exceptions for leap seconds Systems that implement the 32 bit and 64 bit versions of the Windows API such as Windows 9x and Windows NT provide the system time as both SYSTEMTIME represented as a year month day hour minute second milliseconds value and FILETIME represented as a count of the number of 100 nanosecond ticks since 1 January 1601 00 00 00 UT as reckoned in the proleptic Gregorian calendar System time can be converted into calendar time which is a form more suitable for human comprehension For example the Unix system time 1000 000 000 seconds since the beginning of the epoch translates into the calendar time 9 September 2001 01 46 40 UT Library subroutines that handle such conversions may also deal with adjustments for time zones daylight saving time DST leap seconds and the user s locale settings Library routines are also generally provided that convert calendar times into system times Contents 1 Other time measurements 2 History 3 Implementation 4 Retrieving system time 4 1 Operating systems 4 2 Programming languages and applications 5 See also 6 Notes 7 References 8 External linksOther time measurements EditClosely related to system time is process time which is a count of the total CPU time consumed by an executing process It may be split into user and system CPU time representing the time spent executing user code and system kernel code respectively Process times are a tally of CPU instructions or clock cycles and generally have no direct correlation to wall time File systems keep track of the times that files are created modified and or accessed by storing timestamps in the file control block or inode of each file and directory History EditMost first generation personal computers did not keep track of dates and times These included systems that ran the CP M operating system as well as early models of the Apple II the BBC Micro and the Commodore PET among others Add on peripheral boards that included real time clock chips with on board battery back up were available for the IBM PC and XT but the IBM AT was the first widely available PC that came equipped with date time hardware built into the motherboard Prior to the widespread availability of computer networks most personal computer systems that did track system time did so only with respect to local time and did not make allowances for different time zones With current technology most modern computers keep track of local civil time as do many other household and personal devices such as VCRs DVRs cable TV receivers PDAs pagers cell phones fax machines telephone answering machines cameras camcorders central air conditioners and microwave ovens Microcontrollers operating within embedded systems such as the Raspberry Pi Arduino and other similar systems do not always have internal hardware to keep track of time Many such controller systems operate without knowledge of the external time Those that require such information typically initialize their base time upon rebooting by obtaining the current time from an external source such as from a time server or external clock or by prompting the user to manually enter the current time Implementation EditThe system clock is typically implemented as a programmable interval timer that periodically interrupts the CPU which then starts executing a timer interrupt service routine This routine typically adds one tick to the system clock a simple counter and handles other periodic housekeeping tasks preemption etc before returning to the task the CPU was executing before the interruption Retrieving system time Edit19 May 2023 08 06 26 UTCThe Wikipedia system time when this page was last generated Purge this page and update this counter The following tables illustrate methods for retrieving the system time in various operating systems programming languages and applications Values marked by are system dependent and may differ across implementations All dates are given as Gregorian or proleptic Gregorian calendar dates Note that the resolution of an implementation s measurement of time does not imply the same precision of such measurements For example a system might return the current time as a value measured in microseconds but actually be capable of discerning individual clock ticks with a frequency of only 100 Hz 10 ms Operating systems Edit Operating system Command or function Resolution Epoch or rangeAndroid java lang wbr System currentTimeMillis 1 ms 1 January 1970BIOS IBM PC INT 1Ah AH 00h 1 54 9254 ms 18 2065 Hz Midnight of the current dayINT 1Ah AH 02h 2 1 s Midnight of the current dayINT 1Ah AH 04h 3 1 day 1 January 1980 to 31 December 1999 or 31 December 2079 system dependent CP M Plus System Control Block 4 scb base 58h Days since 31 December 1977 scb base 5Ah Hour BCD scb base 5Bh Minute BCD scb base 5Ch Second BCD 1 s 31 December 1977 to 5 June 2157BDOS function 69h gt T GET 5 word Days since 1 January 1978 byte Hour BCD byte Minute BCD byte Second BCD DOS Microsoft samp style padding left 0 4em padding right 0 4em color 666666 C gt samp a href DATE command html class mw redirect title DATE command DATE a br samp style padding left 0 4em padding right 0 4em color 666666 C gt samp a href TIME command html title TIME command TIME a 10 ms 1 January 1980 to 31 December 2099INT 21h AH 2Ch SYSTEM TIME 6 INT 21h AH 2Ah SYSTEM DATE 7 iOS Apple CFAbsoluteTimeGetCurrent 8 lt 1 ms 1 January 2001 10 000 yearsmacOS CFAbsoluteTimeGetCurrent 9 lt 1 ms 10 note 1 1 January 2001 10 000 years 10 note 1 OpenVMS SYS GETTIM 100 ns 11 17 November 1858 to 31 July 31 086 12 span class plainlinks span class plainlinksneverexpand code gettimeofday span span code td 1 ms 13 1 January 1970 to 7 February 2106 14 tr span class plainlinks span class plainlinksneverexpand code clock gettime span span code td 1 ns 13 tr z OS STCK 15 7 187 2 12 ms 244 14 ps 15 4 45 4 46 1 January 1900 to 17 September 2042 UT 16 STCKE 1 January 1900 to AD 36 765 17 Unix POSIX see also C date and time functions samp style padding left 0 4em padding right 0 4em color 666666 samp a href Date Unix html class mw redirect title Date Unix date a a href Time html class mw redirect title Time time a 1 s 1 January 1970 to 19 January 2038 prior to Linux 5 9 to 2 July 2486 Since Linux 5 10 1 January 1970 to 4 December AD 292 277 026 596 span class plainlinks span class plainlinksneverexpand code gettimeofday span span code td 1 ms tr span class plainlinks span class plainlinksneverexpand code clock gettime span span code td 1 ns tr OS 2 DosGetDateTime 10 ms 1 January 1980 to 31 December 2079 18 Windows GetSystemTime 1 ms 1 January 1601 to 14 September 30828 02 48 05 4775807GetSystemTimeAsFileTime 100 nsGetSystemTimePreciseAsFileTime Programming languages and applications Edit Language Application Function or variable Resolution Epoch or rangeAda Ada Calendar Clock 100 ms to 20 ms 1 January 1901 to 31 December 2099 AWK systime 1 s BASIC True BASIC DATE DATE TIME TIME 1 s Business BASIC DAY TIM 0 1 s C see C date and time functions a href Time html class mw redirect title Time time a 1 s note 2 note 2 C std time std chrono system clock now 1 s note 2 1 ns C 11 OS dependent note 2 C System DateTime Now 19 System DateTime UtcNow 20 100 ns 21 1 January 0001 to 31 December 9999CICS ASKTIME 1 ms 1 January 1900COBOL FUNCTION CURRENT DATE 1 s 1 January 1601Common Lisp get universal time 1 s 1 January 1900Delphi Borland datetime 1 ms floating point 1 January 1900Delphi Embarcadero Technologies 22 System SysUtils Time 23 1 ms 0 0 0000 0 0 0 000 to 12 31 9999 23 59 59 999 sic System SysUtils GetTime 24 alias for System SysUtils Time System SysUtils Date 25 0 0 0000 0 0 0 000 to 12 31 9999 0 0 0 000 sic System DateUtils Today 26 System DateUtils Tomorrow 27 System DateUtils Yesterday 28 System SysUtils Now 29 1 s 0 0 0000 0 0 0 000 to 12 31 9999 23 59 59 000 sic System SysUtils DayOfWeek 30 1 day 1 to 7System SysUtils CurrentYear 31 1 year Emacs Lisp current time 1 ms 1 January 1970Erlang erlang system time os system time 32 OS dependent e g on Linux 1ns 32 1 January 1970 32 Excel date 0 January 1900 33 Fortran DATE AND TIME SYSTEM CLOCK 34 35 1 January 1970CPU TIME 1 msGo time Now 1 ns 1 January 0001Haskell Time getClockTime 1 ps 1 January 1970 Data Time getCurrentTime 1 ps 17 November 1858 Java java util Date System currentTimeMillis 1 ms 1 January 1970System nanoTime 36 1 ns arbitrary 36 Clock systemUTC 37 1 ns arbitrary 38 JavaScript TypeScript new Date getTime Date now 1 ms 1 January 1970Matlab now 1 s 0 January 0000 39 MUMPS H short for a href Horology html title Horology HOROLOG a 1 s 31 December 1840LabVIEW Tick Count 1 ms 00 00 00 000 1 January 1904Get Date Time in Seconds 1 ms 00 00 00 000 1 January 1904Objective C NSDate timeIntervalSinceReferenceDate lt 1 ms 40 1 January 2001 10 000 Years 40 OCaml Unix time 1 s 1 January 1970Unix gettimeofday 1 msExtended Pascal GetTimeStamp 1 s Turbo Pascal GetTime GetDate 10 ms Perl time 1 s 1 January 1970Time HiRes time 41 1 msPHP time mktime 1 s 1 January 1970microtime 1 msPureBasic Date 1 s 1 January 1970 to 19 January 2038Python datetime now timestamp 1 ms 1 January 1970RPG CURRENT DATE DATE CURRENT TIME TIME 1 s 1 January 0001 to 31 December 9999CURRENT TIMESTAMP TIMESTAMP 1 msRuby Time now 42 1 ms 1 January 1970 to 19 January 2038 prior to Ruby 1 9 2 43 Smalltalk Time microsecondClock VisualWorks 1 s ANSI 1 ms VisualWorks 1 s Squeak 1 January 1901 Time totalSeconds Squeak SystemClock ticksNowSinceSystemClockEpoch Chronos SQL CURDATE or CURRENT DATE CURTIME or CURRENT TIME GETDATE NOW or CURRENT TIMESTAMP SYSDATE 3 ms 1 January 1753 to 31 December 9999 60 s 1 January 1900 to 6 June 2079Standard ML Time now 1 ms 1 January 1970 TCL clock seconds 1 s 1 January 1970 clock milliseconds 1 ms clock microseconds 1 ms clock clicks 1 ms Windows PowerShell Get Date 44 45 100 ns 21 1 January 0001 to 31 December 9999 DateTime Now 19 DateTime UtcNow 20 Visual Basic NET System DateTime Now 19 System DateTime UtcNow 20 100 ns 21 1 January 0001 to 31 December 9999See also EditCalendar DATE command Epoch GPS time Jiffy Network Time Protocol rdate TIME command time h time t Time formatting and storage bugs Time Protocol Time standard Timestamp Unix date command Unix time Unix time command Elapsed real time Year 2000 problem Year 2038 problemNotes Edit a b The Apple Developer Documentation is not clear on the precision amp range of CFAbsoluteTime CFTimeInterval except in the CFRunLoopTimerCreate documentation which refers to sub millisecond at most precision However the similar type NSTimeInterval appears to be interchangeable and has the precision and range listed a b c d The C standard library does not specify any specific resolution epoch range or datatype for system time values The C library encompasses the C library so it uses the same system time implementation as C References Edit Ralf D Brown 2000 Int 0x1A AH 0x00 Ralf Brown s Interrupt List Ralf D Brown 2000 Int 0x1A AH 0x02 Ralf Brown s Interrupt List Ralf D Brown 2000 Int 0x1A AH 0x04 Ralf Brown s Interrupt List CP M Plus CP M Version 3 0 Operating System Guide PDF BDOS system calls Ralf D Brown 2000 Int 0x21 AH 0x2c Ralf Brown s Interrupt List Ralf D Brown 2000 Int 0x21 AH 0x2a Ralf Brown s Interrupt List Time Utilities Reference iOS Developer Library 2007 Time Utilities Reference Mac OS X Developer Library 2007 a b Time Utilities Foundation Apple Developer Documentation Retrieved 6 July 2022 Ruth E Goldenberg Lawrence J Kenah Denise E Dumas 1991 VAX VMS Internals and Data Structures Version 5 2 Digital Press ISBN 978 1555580599 Why is Wednesday November 17 1858 the base time for OpenVMS VAX VMS Stanford University 24 July 1997 Archived from the original on 24 July 1997 Retrieved 8 January 2020 a b VSI C Run Time Library Reference Manual for OpenVMS Systems PDF VSI November 2020 Retrieved 2021 04 17 OpenVMS and the year 2038 HP Retrieved 2021 04 17 a b z Architecture Principles of Operation PDF Poughkeepsie New York International Business Machines 2007 IBM intends to extend the date range on future systems beyond 2042 z Architecture Principles of Operation Poughkeepsie New York International Business Machines 2007 1 15 4 45 to 4 47 Expanded 64 bit time values IBM Retrieved 2021 04 18 Jonathan de Boyne Pollard The 32 bit Command Interpreter On OS 2 Warp 4 date and time can both operate well beyond the year 2000 and even well beyond the year 2038 and in fact up to the year 2079 which is the limit for OS 2 Warp 4 s real time clock a b c DateTime Now Property Microsoft Docs a b c DateTime UtcNow Property Microsoft Docs a b c DateTime Ticks Property Microsoft Docs Date and Time Support Embarcadero Developer Network 2013 System SysUtils Time Embarcadero Developer Network 2013 System SysUtils GetTime Embarcadero Developer Network 2013 System SysUtils Date Embarcadero Developer Network 2013 System DateUtils Today Embarcadero Developer Network 2013 System DateUtils Tomorrow Embarcadero Developer Network 2013 System DateUtils Yesterday Embarcadero Developer Network 2013 System SysUtils Now Embarcadero Developer Network 2013 System SysUtils DayOfWeek Embarcadero Developer Network 2013 System SysUtils CurrentYear Embarcadero Developer Network 2013 a b c Time and Time Correction in Erlang www erlang org XL2000 Early Dates on Office Spreadsheet Component Differ from Excel Microsoft Support 2003 Archived from the original on 24 October 2007 In the Microsoft Office Spreadsheet Component the value 0 evaluates to the date December 30 1899 and the value 1 evaluates to December 31 1899 In Excel the value 0 evaluates to January 0 1900 and the value 1 evaluates to January 1 1900 SYSTEM CLOCK Intel Fortran Compiler 19 0 Developer Guide and Reference 29 April 2019 Retrieved 23 July 2019 SYSTEM CLOCK Time function The GNU Fortran Compiler Retrieved 27 October 2011 a b System nanoTime method Java Platform Standard Edition 6 API Specification 2015 Retrieved 23 July 2019 Clock systemUTC and other methods Java Platform Standard Edition 8 API Specification 2014 Retrieved 15 January 2015 JSR 310 Java Time System Java Platform Standard Edition 8 API Specification 2014 Retrieved 15 January 2015 Matlab Help a b NSTimeInterval Foundation Apple Developer Documentation Douglas Wegscheild R Schertler and Jarkko Hietaniemi Time HiRes CPAN Comprehensive Perl Archive Network 2011 Retrieved 27 October 2011 James Britt Neurogami Time class Ruby Doc org Help and documentation for the Ruby programming language Scottsdale AZ Retrieved 27 October 2011 Yugui 18 August 2010 Ruby 1 9 2 is released The new 1 9 2 is almost compatible with 1 9 1 except these changes Time is reimplemented The bug with year 2038 is fixed Using the Get Date Cmdlet Microsoft Docs Retrieved 23 July 2019 Windows PowerShell Tip of the Week Formatting Dates and Times Microsoft Docs Retrieved 23 July 2019 External links EditCritical and Significant Dates J R Stockton retrieved 3 December 2015 The Boost Date Time Library C The Boost Chrono Library C The Chronos Date Time Library Smalltalk Joda Time The Joda Date Time Library Java The Perl DateTime Project Archived 2009 02 19 at the Wayback Machine Perl date Ruby Standard Library Documentation Ruby Retrieved from https en wikipedia org w index php title System time amp oldid 1146581532, wikipedia, wiki, book, books, library,
