Unix date command

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 ("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, but returns the current time to the nearest millisecond^{[citation needed]}.

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[link]

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.

History[link]

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.

The system clock is often implemented as a programmable interval timer that periodically interrupts the CPU, which then starts executing the timer interrupt service routine. That routine adds one tick to the system clock and handles other periodic housekeeping tasks -- preemption, etc. -- before returning to whatever the CPU was doing before the interruption.

Retrieving system time[link]

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.

Operating systems[link]

Programming languages and applications[link]

Notes[link]

1. ^ ^{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.
2. ^ ^{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.

See also[link]

Calendar Computation time Epoch GPS time Jiffy Network Time Protocol Process rdate time.h time_t time and date Time formatting and storage bugs

TIME protocol Time-sharing Time standard Timer Timestamp Unix date command Unix time Unix time command Wall clock time Year 2000 problem Year 2038 problem

Time portal

References[link]

External links[link]

• Critical and Significant Dates, J. R. Stockton (retrieved 1 September 2006)
• 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 (Perl)
• The Ruby Date/Time Library (Ruby)

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