- System time
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 as the number of seconds elapsed since the start of the Unix epoch at 1 January 1970 00:00:00 UT. Windows NT counts the number of 100-nanosecond ticks since 1 January 1601 00:00:00 UT as reckoned in the proleptic Gregorian calendar, but returns the current time to the nearest millisecond.
System time can be converted into calendar time, which is a form more suitable for human comprehension. For example, the Unix system time 1000000000 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 timezones, 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.
Other time measurements
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 PCs did not keep track of dates and times. These included systems that ran the CP/M operating system, early models of the Apple II, and the Commodore PET, among others. The IBM PC was the first widely available personal computer that came equipped with date/time hardware built into the motherboard, and subsequent add-on peripheral boards included real-time clock chips with on-board battery back-up. 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 other time zones.
With current technology, all 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.
Retrieving system time
Operating system Command or function Resolution Epoch or range BIOS (IBM PC) INT 1Ah,AH=00h 54.931 ms
Midnight of the current day INT 1Ah,AH=02h</ref> 1 s 1 January 1980 DOS (Microsoft) TIME
10 ms 1 January 1980 to 31 December 2099 Mac OS (Apple) CFAbsoluteTimeGetCurrent() < 1 ms [note 1] 1 January 2001 ±10,000 years[note 1] OpenVMS (HP) SYS$GETTIM() 100 ns 17 November 1858 to AD 31,086 z/OS (IBM) STCK 2−12 μs
1 January 1900 to 17 September 2042 UT Unix, POSIX
1 s (*)
1 January 1970 to 19 January 2038
1 January 1970 to AD 292,277,026,596
() 1 μs Windows (Microsoft) GetSystemTime() 1 ms 1 January 1601 to AD 30,828 GetSystemTimeAsFileTime() 100 ns
Programming languages and applications
Language/Application Function or variable Resolution Epoch or range Ada Ada.Calendar.Clock 100 μs to
20 ms (*)
1 January 1901 to 31 December 2099 (*) AWK systime() 1 s (*) BASIC, True BASIC DATE, DATE$
1 s (*) Business BASIC DAY, TIM 0.1 s (*) C time() 1 s (*)[note 2] (*)[note 2] C++ std::time() 1 s (*)[note 2] (*)[note 2] C# (Microsoft) System.DateTime.Now
100 ns  1 January 1 to 31 December 9999 CICS (IBM) ASKTIME 1 ms 1 January 1900 COBOL FUNCTION CURRENT-DATE 1 s 1 January 1601 Common Lisp (get-universal-time) 1 s 1 January 1900 Delphi (Borland) date
1 January 1900 Excel (Microsoft) date() ? 0 January 1900 Fortran DATE_AND_TIME
(*) 1 January 1970 CPU_TIME 1 μs Haskell Time.getClockTime 1 μs (*) 1 January 1970 (*) Java (Sun) java.util.Date()
10 ms (*) Perl time() 1 s 1 January 1970 Time::HiRes::time 1 μs PHP time()
1 s 1 January 1970 microtime() 1 μs Python time.time() 1 μs (*) 1 January 1970 RPG CURRENT(DATE), %DATE
1 s 1 January 0001 to 31 December 9999 CURRENT(TIMESTAMP), %TIMESTAMP 1 μs Ruby Time.now() 1 μs (*) 1 January 1970 (to 19 January 2038 prior to Ruby 1.9.2) Smalltalk Time microsecondClock
1 s (ANSI)
1 μs (VisualWorks)
1 s (Squeak)
1 January 1901 (*) Time totalSeconds
3 ms 1 January 1753 to 31 December 9999 (*) 60 s 1 January 1900 to 6 June 2079 Standard ML Time.now () 1 μs (*) 1 January 1970 (*) TCL [clock seconds] 1 s 1 January 1970 [clock milliseconds] 1 ms [clock microseconds] 1 μs [clock clicks] 1 μs (*) (*) Windows PowerShell Get-Date 100 ns  1 January 1 to 31 December 9999 [DateTime]::Now
Visual Basic .NET (Microsoft) System.DateTime.Now
100 ns  1 January 1 to 31 December 9999
- ^ a b The 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.
- ^ 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.
- ^ Ralf Brown, "Int 0x1A, AH=0x00" in Ralf Brown's Interrupt List, 2000, http://www.delorie.com/djgpp/doc/rbinter/ix/1A/00.html
- ^ Ralf Brown, "Int 0x1A, AH=0x02" in Ralf Brown's Interrupt List, 2000, http://www.delorie.com/djgpp/doc/rbinter/ix/1A/02.html
- ^ Ralf Brown, "Int 0x21, AH=0x2c" in Ralf Brown's Interrupt List, 2000, http://www.delorie.com/djgpp/doc/rbinter/ix/21/2C.html
- ^ "Time Utilities Reference" in Mac OS X Developer Library (Apple, 2007).
- ^ a b "CFRunLoopTimer Reference" in Mac OS X Developer Library (Apple, 2007).
- ^ z/Architecture Principles of Operation (Poughkeepsie, New York:International Business Machines, 2007) 7-187.
- ^ z/Architecture Principles of Operation, (Poughkeepsie, New York:International Business Machines, 2000) 4-45, 4-46.
- ^ 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.
- ^ a b c "DateTime.Now Property" in MSDN (Microsoft, 2010) last updated July 2010.
- ^ a b c "DateTime.UtcNow Property" in MSDN (Microsoft, 2011)
- ^ a b c "DateTime.Ticks Property in MSDN (Microsoft, 2010) last updated May 2010.
- ^ "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." XL2000: Early Dates on Office Spreadsheet Component Differ from Excel in (Microsoft Support, 2003).
- ^ "SYSTEM_CLOCK", documentation for FORTRAN compiler, Intel Corp., accessed 27 October 2011.
- ^ SYSTEM_CLOCK — Time function" in The GNU Fortran Compiler (Free Software Foundation) accessed 27 October 2011.
- ^ a b System.nanoTime() method in Java Platform, Standard Edition 6: API Specification (Oracle, 2011) accessed 27 October 2011.
- ^ a b "Foundation Data Types Reference" in Mac OS X Developer Library (Apple, 2011) last modified 6 July 2011, section NSTimeInterval.
- ^ Douglas Wegscheild, R. Schertler, and Jarkko Hietaniemi, "Time::HiRes" (CPAN Comprehensive Perl Archive Network, 2011) accessed 27 October 2011.
- ^ Time class in Ruby-Doc.org: Help and documentation for the Ruby programming language (Scottsdale, AZ: James Britt and Neurogami) accessed 27 October 2011.
- ^ Ruby 1.9.2 Release Notes] in Ruby-Doc.org: Help and documentation for the Ruby programming language (Scottsdale, AZ: James Britt and Neurogami) accessed 27 October 2011.
- ^ "Get-Date" in Microsoft TechNet,[dead link].
- Critical and Significant Dates, J. R. Stockton (retrieved 1 September 2006)
- The BOOST Date/Time Library (C++)
- The Chronos Date/Time Library (Smalltalk)
- Joda Time, The Joda Date/Time Library (Java)
- The Perl DateTime Project (Perl)
- The Ruby Date/Time Library (Ruby)
Time Major concepts
Measurement and standardsChronometry · UTC · UT · TAI · Second · Minute · Hour · Sidereal time · Solar time · Time zone
Clock · Astrarium · History of timekeeping devices · Horology · Marine chronometer · Sundial · Water clock
Calendar · Day · Week · Month · Year · Tropical year · Gregorian · Islamic · Julian
Intercalation · Leap second · Leap year
Chronology Religion and mythology Philosophy Physical sciences Biology Psychology Sociology and anthropology Economics Related topics Time measurement and standards Major subjects International standards Obsolete standards Time in physics Horology Calendar Archaeology & geology Astronomical chronology Units of time Related topics
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