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 1970-01-01 00:00:00 Z. Windows NT counts the number of 100-nanosecond ticks since 1601-01-01 00:00:00 Z 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 2001-09-09 01:46:40 UTC (sans leap seconds). 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 wall 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
| 2009-07-07 21:11:12 Z This was the Wikipedia system time when this page was last generated. |
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.
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 | 1980-01-01[2] | |
| DOS (Microsoft) | TIME INT 21h,AH=2Ch[1] |
10 ms | 1980-01-01 to 2099-12-31 |
| Mac OS (Apple) | GetDateTime() | 1 s | 1904-01-01 to 2040-02-06 |
| OpenVMS (HP) | SYS$GETTIM() | 100 ns | 1858-11-17 to AD 31,086 |
| z/OS (IBM) | STCK | 2−12 μs 244.14 ps |
1900-01-01 to 2042-09-17 |
| Unix, POSIX | () |
1 s | (*) 1970-01-01 to 2038-01-19 1970-01-01 to AD 292,277,026,596 |
| () | 1 μs | ||
| Windows (Microsoft) | GetSystemTime() | 1 ms | 1601-01-01 to AD 30,828 |
| GetSystemTimeAsFileTime() | 100 ns |
Programming languages
| Programming Language | Function / Variable | Resolution | Epoch / Range |
|---|---|---|---|
| Ada | Ada.Calendar.Clock | 100 μs to 20 ms (*) |
1901-01-01 to 2099-12-31 (*) |
| AWK | systime() | 1 s | (*) |
| BASIC, True BASIC | DATE, DATE$ TIME, TIME$ |
1 s | (*) |
| C | time() | 1 s(*) | (*) |
| C++ | std::time() | 1 s(*) | (*) |
| C# (Microsoft) | System.DateTime.Now[3] System.DateTime.UtcNow[4] |
100 ns [5] | 0001-01-01 to 9999-12-31 |
| CICS (IBM) | ASKTIME | 1 ms | 1900-01-01 |
| COBOL | FUNCTION CURRENT-DATE | 1 s | 1601-01-01 |
| Common Lisp | (get-universal-time) | 1 s | 1900-01-01 |
| Delphi (Borland) | date time |
1 ms (floating point) |
1900-01-01 |
| FORTRAN | TIME ITIME, IDATE |
1 s | (*) |
| Haskell | Time.getClockTime | 1 μs(*) | 1970-01-01(*) |
| Java (Sun) | java.util.Date() System.currentTimeMillis() |
1 ms | 1970-01-01 |
| System.nanoTime()[6] | 1 μs (*) | ||
| JavaScript | Date() | 1 ms | 1970-01-01 |
| MUMPS | $H (short for $HOROLOG) | 1 s | 1840-12-31 |
| Objective-C | [NSDate timeIntervalSinceReferenceDate] | 1 μs? | 2001-01-01 |
| OCaml | Unix.time () | 1 s | 1970-01-01 |
| Unix.gettimeofday () | 1 μs | ||
| Extended Pascal | GetTimeStamp() | 1 s | (*) |
| Turbo Pascal | GetTime() GetDate() |
10 ms | (*) |
| Perl | time() | 1 s | 1970-01-01 |
| Time::HiRes::time[7] | 1 μs | ||
| PHP | time() mktime() |
1 s | 1970-01-01 |
| microtime() | 1 μs | ||
| Python | time.time() | 1 μs (*) | 1970-01-01 |
| Ruby | Time.now()[8] | 1 μs (*) | 1970-01-01 to 2038-01-19 |
| Smalltalk | Time microsecondClock (VisualWorks) |
1 s (ANSI) 1 μs (VisualWorks) 1 s (Squeak) |
1901-01-01 (*) |
| Time totalSeconds (Squeak) |
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| SystemClock ticksNowSinceSystemClockEpoch (Chronos) |
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| SQL | CURDATE() CURTIME() GETDATE() NOW() SYSDATE() |
3 ms | 1753-01-01 to 9999-12-31 (*) |
| 60 s | 1900-01-01 to 2079-06-06 | ||
| Standard ML | Time.now () | 1 μs(*) | 1970-01-01(*) |
| TCL | [clock seconds] | 1 s | 1970-01-01 |
| [clock milliseconds] | 1 ms | ||
| [clock microseconds] | 1 μs | ||
| [clock clicks] | 1 μs (*) | (*) | |
| Windows PowerShell | Get-Date[9] | 100 ns [5] | 0001-01-01 to 9999-12-31 |
| [DateTime]::Now[3] [DateTime]::UtcNow[4] |
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| Visual Basic .NET (Microsoft) | System.DateTime.Now[3] System.DateTime.UtcNow[4] |
100 ns [5] | 0001-01-01 to 9999-12-31 |
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.
- ^ 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
- ^ 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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