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. Microsoft Windows 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.
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.
Retrieving system time
According to the Wikipedia system clock, the current time is 17:36, Saturday November 3 2007 (UTC).
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 System | Command or Function | Resolution | Epoch |
|---|---|---|---|
| DOS (Microsoft) | date time |
1 sec | January 1, 1980 to January 1, 2108 |
| INT 1Ah | 54.931 msec 18.204 Hz |
Recycled every 60 min | |
| INT 70h | 976.6 msec 1,024 Hz |
Reset on reboot | |
| Mac OS (Apple) | GetDateTime() | 1 sec | January 1, 1904 to February 6, 2040 |
| OpenVMS (HP) | SYS$GETTIM() | 100 nsec | November 17, 1858 to AD 31086 |
| z/OS (IBM) | STCK | 2−12 μsec 244.14 picosec |
January 1, 1900 to September 17, 2042 |
| Unix, POSIX | date time() |
1 sec | (*) January 1, 1970 to January 19, 2038 January 1, 1970 to AD 292277026596 |
| gettimeofday() | 1 μsec | ||
| Windows (Microsoft) | GetSystemTime() | 1 msec | January 1, 1601 to AD 30828 |
| GetSystemTimeAsFileTime() | 100 nsec |
| Programming Language | Function or Variable | Resolution | Epoch |
|---|---|---|---|
| Ada | Ada.Calendar.Clock | 100 μsec to 20 msec (*) |
January 1, 1901 to December 31, 2099 (*) |
| AWK | systime() | 1 sec | (*) |
| BASIC, True BASIC | DATE, DATE$ TIME, TIME$ |
1 sec | (*) |
| C | time() | 1 sec (*) | (*) |
| [[C++]] | std::time() | 1 sec (*) | (*) |
| CICS (IBM) | ASKTIME | 1 msec | January 1, 1900 |
| COBOL | FUNCTION CURRENT-DATE | 1 sec | January 1, 1601 |
| Delphi (Borland) | date time |
1 msec (floating point) |
January 1, 1900 |
| FORTRAN | TIME ITIME, IDATE |
1 sec | (*) |
| Java | java.util.Date() | 1 msec | January 1, 1970 |
| MUMPS | $H (short for $HOROLOG) | 1 sec | December 31, 1840 |
| Extended Pascal | GetTimeStamp() | 1 sec | (*) |
| Turbo Pascal | GetTime() GetDate() |
10 msec | (*) |
| Perl | time() | 1 sec | January 1, 1970 |
| PHP | time() mktime() |
1 sec | January 1, 1970 |
| microtime() | 1 msec | ||
| Python | time.time() | 1 μsec (*) | January 1, 1970 |
| Smalltalk | Time microsecondClock (VisualWorks) |
1 sec (ANSI) 1 μsec (VisualWorks) 1 sec (Squeak) |
January 1, 1901 (*) |
| Time totalSeconds (Squeak) |
|||
| SystemClock ticksNowSinceSystemClockEpoch (Chronos) |
|||
| SQL | CURDATE() CURTIME() NOW() SYSDATE() |
3 msec | January 1, 1753 to December 31, 9999 (*) |
| 60 sec | January 1, 1900 to June 6, 2079 | ||
| TCL | [clock seconds] | 1 sec | January 1, 1970 |
| [clock milliseconds] | 1 msec | ||
| [clock microseconds] | 1 μsec | ||
| [clock clicks] | 1 μsec (*) | (*) |
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)
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
