In computer science and computer programming, system time represents a computer system's notion of the passing of time. In this sense, time also includes the passing of days on the calendar.
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 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 that is 1,000,000,000 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 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.
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.
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.
It should be noted that most first-generation PCs did not keep track of dates and times. These included systems that ran the CP/M operating system, 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.
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Retrieving system time
The following tables illustrate methods for retrieving the system time in various operating systems and programming languages.
Note: values marked by (*) are system-dependent and may differ across implementations.
All dates are given as Gregorian calendar or proleptic Gregorian calendar dates.
Operating systems
| Operating System | Command / Function | Resolution | Epoch / Range |
|---|---|---|---|
| BIOS (IBM PC) | INT 1Ah,AH=00h[1] | 54.931 ms 18.204 Hz |
Midnight of the current day |
| INT 1Ah,AH=02h[1] | 1 s | 1 January 1980[2] | |
| DOS (Microsoft) | TIME INT 21h,AH=2Ch[1] |
10 ms | 1 January 1980 to 31 December 2099 |
| Mac OS (Apple) | CFAbsoluteTimeGetCurrent()[3] | < 1 ms [4][note 1] | 1 January 2001 ±10,000 years[4][note 1] |
| OpenVMS (HP) | SYS$GETTIM() | 100 ns | 17 November 1858 to AD 31,086 |
| z/OS (IBM) | STCK | 2−12 μs 244.14 ps |
1 January 1900 to 17 September 2042 |
| Unix, POSIX | () |
1 s | (*) 1 January 1970 to 19 January 2038 1 January 1970 to AD 292,277,026,596 |
| () | 1 μs | ||
| Windows (Microsoft) | GetSystemTime() | 1 ms | 1 January 1601 to AD 30,828 |
| GetSystemTimeAsFileTime() | 100 ns |
Programming languages
| Programming Language | Function / Variable | Resolution | Epoch / Range |
|---|---|---|---|
| Ada | Ada.Calendar.Clock | 100 μs to 20 ms (*) |
1 January 1901 to 31 December 2099 (*) |
| AWK | systime() | 1 s | (*) |
| BASIC, True BASIC | DATE, DATE$ TIME, TIME$ |
1 s | (*) |
| C | time() | 1 s (*) | (*) |
| C++ | std::time() | 1 s (*) | (*) |
| C# (Microsoft) | System.DateTime.Now[5] System.DateTime.UtcNow[6] |
100 ns [7] | 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 time |
1 ms (floating point) |
1 January 1900 |
| FORTRAN | TIME ITIME, IDATE |
1 s | (*) |
| Haskell | Time.getClockTime | 1 μs (*) | 1 January 1970 (*) |
| Java (Sun) | java.util.Date() System.currentTimeMillis() |
1 ms | 1 January 1970 |
| System.nanoTime()[8] | 1 ns | ||
| JavaScript | Date() | 1 ms | 1 January 1970 |
| MUMPS | $H (short for $HOROLOG) | 1 s | 31 December 1840 |
| Objective-C | [NSDate timeIntervalSinceReferenceDate] | < 1 ms[9] | 1 January 2001 ±10,000 Years[9] |
| OCaml | Unix.time () | 1 s | 1 January 1970 |
| Unix.gettimeofday () | 1 μs | ||
| Extended Pascal | GetTimeStamp() | 1 s | (*) |
| Turbo Pascal | GetTime() GetDate() |
10 ms | (*) |
| Perl | time() | 1 s | 1 January 1970 |
| Time::HiRes::time[10] | 1 μs | ||
| PHP | time() mktime() |
1 s | 1 January 1970 |
| microtime() | 1 μs | ||
| Python | time.time() | 1 μs (*) | 1 January 1970 |
| Ruby | Time.now()[11] | 1 μs (*) | 1 January 1970 to 19 January 2038 |
| Smalltalk | Time microsecondClock (VisualWorks) |
1 s (ANSI) 1 μs (VisualWorks) 1 s (Squeak) |
1 January 1901 (*) |
| Time totalSeconds (Squeak) |
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| SystemClock ticksNowSinceSystemClockEpoch (Chronos) |
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| SQL | CURDATE() CURTIME() GETDATE() NOW() SYSDATE() |
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[12] | 100 ns [7] | 1 January 1 to 31 December 9999 |
| [DateTime]::Now[5] [DateTime]::UtcNow[6] |
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| Visual Basic .NET (Microsoft) | System.DateTime.Now[5] System.DateTime.UtcNow[6] |
100 ns [7] | 1 January 1 to 31 December 9999 |
Notes
- ^ 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.
References
- ^ a b c Ralf Brown's Interrupt List; online browsable version at [1]
- ^ Y2K bug of the original RTC (also known as CMOS) chip here; since about 1998 any BIOS which "sees" century (CMOS register 32h) as '19' while year (CMOS register 09h) is less than '80' will automatically adjust the century upward on boot; as a result it is now impossible to set up the RTC clock before 1980.
- ^ Online reference at developer.apple.com
- ^ a b Online reference at developer.apple.com
- ^ a b c System.DateTime.Now at microsoft.com
- ^ a b c System.DateTime.UtcNow at microsoft.com
- ^ a b c MSDN link to DateTime.Ticks[2]
- ^ System.nanoTime() method at java.sun.com
- ^ a b NSTimeInterval
- ^ Jarkko Hietaniemi / Time-HiRes - search.cpan.org
- ^ Time class at ruby-doc.org
- ^ Get-Date
See also
External links
- Critical and Significant Dates, J. R. Stockton (retrieved 2006-09-01)
- 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)
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