NationMaster - Encyclopedia: System time

In computer science and computer programming, system time represents a computer system's notion of the passing of time. Computer science, or computing science, is the study of the theoretical foundations of information and computation and their implementation and application in computer systems. ... For programming in music, see Programming (music). ...

System time 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 one-second ticks 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. In chronology, an epoch is an instant chosen as the origin of a particular time scale. ... Unix or UNIX is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T Bell Labs employees including Ken Thompson, Dennis Ritchie, and Douglas McIlroy. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Unix time, or POSIX time, is a system for describing points in time. ... UTC also stands for the University of Tennessee at Chattanooga Coordinated Universal Time or UTC, also sometimes referred to as Zulu time, the basis for civil time, differs by an integral number of seconds from atomic time and a fractional number of seconds from UT1. ... Microsoft Windows is a family of operating systems by Microsoft for use on personal computers, although versions of Windows designed for servers, embedded devices, and other platforms also exist. ... UTC also stands for the University of Tennessee at Chattanooga Coordinated Universal Time or UTC, also sometimes referred to as Zulu time, the basis for civil time, differs by an integral number of seconds from atomic time and a fractional number of seconds from UT1. ... A Proleptic calendar or era is that calendar extrapolated to dates prior the its first adoption. ... This article or section does not cite its references or sources. ...

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. 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. A date in a calendar is a reference to a particular day by means of a calendar system. ... Unix time, or POSIX time, is a system for describing points in time. ... In computer science, a subroutine (function, procedure, or subprogram) is a sequence of code which performs a specific task, as part of a larger program, and is grouped as one, or more, statement blocks; such code is sometimes collected into software libraries. ... A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time. ... Map of the world color-coded with areas in blue observing daylight saving time. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... In computing, locale is a set of parameters that defines the users language, country and any special variant preferences that the user wants to see in their user interface. ...

Closedly 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 instruction cycles and generally have no direct correlation to wall time. In computing, a process is a running instance of a program, including all variables and other states. ... The kernel is the central part in most computer operating systems because of its task, which is the management of the systems resources and the communication between hardware and software components. ...

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. See Filing system for this term as it is used in libraries and offices In computing, a file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. ... A File Control Block is an file system structure in which the state of an open file is maintained. ... In computing, an inode is a data structure on a traditional Unix-style file system such as ext2. ...

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. A stylised illustration of a modern personal computer A personal computer (PC) is usually a microcomputer whose price, size, and capabilities make it suitable for personal usage. ... CP/M is an operating system originally created for Intel 8080/85 and Zilog Z80 based microcomputers by Gary Kildall of Digital Research, Inc. ... The Apple II was one of the most popular personal computers of the 1980s. ... The PET (Personal Electronic Transactor) was a home-/personal computer produced by Commodore starting in the late 1970s. ... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ... A8N VM CSM, Asuss Flagship MicroATX motherboard An old Octek Jaguar V mainboard with an AMD 386DX-40 processor. ... A peripheral is a type of computer hardware that is added to a host computer in order to expand its abilities. ... A real-time clock (RTC) is a computer clock (most often in the form of an integrated circuit chip) that keeps track of the current time even when the computer is turned off. ... Four double-A batteries In science and technology, a battery is a device that stores energy and makes it available in an electrical form. ...

Retrieving System Time

The following tables illustrate methods for retrieving the system time in various operating systems and programming languages.

Note: Values marked (*) are system-dependent and may be different on some implementations. Perl, also Practical Extraction and Report Language (a backronym, see below) is a dynamic procedural programming language designed by Larry Wall and first released in 1987. ... PHP is an open-source, reflective programming language. ...

See also