Incomputer science andcomputer programming,system time represents a computer system's notion of the passage of time. In this sense,time also includes the passing ofdays on the calendar.

System time is measured by asystem clock, which is typically implemented as a simple count of the number ofticks that have transpired since some arbitrary starting date, called theepoch. For example,Unix andPOSIX-compliant systems encode system time ("Unix time") as the number of seconds elapsed since the start of theUnix epoch at 1 January 1970 00:00:00UT, with exceptions forleap seconds. Systems that implement the 32-bit and 64-bit versions of theWindows API, such asWindows 9x andWindows NT, provide the system time as bothSYSTEMTIME, represented as a year/month/day/hour/minute/second/milliseconds value, andFILETIME, represented as a count of the number of 100-nanosecond ticks since 1 January 1601 00:00:00 UT as reckoned in theproleptic Gregorian calendar.

System time can be converted intocalendar time, which is a form more suitable for human comprehension. For example, theUnix system time1000000000 seconds since the beginning of the epoch translates into the calendar time9 September 2001 01:46:40 UT. Librarysubroutines that handle such conversions may also deal with adjustments fortime zones,daylight saving time (DST), leap seconds, and the user'slocale settings. Library routines are also generally provided that convert calendar times into system times.

Many implementations that currently store system times as 32-bit integer values will suffer from the impendingYear 2038 problem. These time values will overflow ("run out of bits") after the end of their system time epoch, leading tosoftware and hardware errors. These systems will require some form of remediation, similar to efforts required to solve the earlierYear 2000 problem. This will also be a potentially much larger problem for existingdata file formats that contain system timestamps stored as 32-bit values.

Closely related to system time isprocess time, which is a count of the totalCPU time consumed by an executingprocess. It may be split intouser andsystem CPU time, representing the time spent executing user code and systemkernel code, respectively. Process times are a tally of CPUinstructions orclock cycles and generally have no direct correlation towall time.

File systems keep track of the times that files are created, modified, and/or accessed by storingtimestamps in thefile control block (orinode) of eachfile anddirectory.

Most first-generationpersonal computers did not keep track of dates and times. These included systems that ran theCP/M operating system, as well as early models of theApple II, theBBC Micro, and theCommodore PET, among others. Add-onperipheral boards that includedreal-time clock chips with on-boardbattery back-up were available for theIBM PC andXT, but theIBM AT was the first widely available PC that came equipped with date/time hardware built into themotherboard. Prior to the widespread availability ofcomputer networks, most personal computer systems that did track system time did so only with respect tolocal time and did not make allowances for differenttime zones.

With current technology, most modern computers keep track of local civil time, as do many other household and personal devices such asVCRs,DVRs,cable TV receivers,PDAs,pagers,cell phones,fax machines,telephone answering machines,cameras,camcorders,central air conditioners, andmicrowave ovens.

Microcontrollers operating withinembedded systems (such as theRaspberry Pi,Arduino, and othersimilar systems) do not always have internal hardware to keep track of time. Many such controller systems operate without knowledge of the external time. Those that require such information typically initialize their base time uponrebooting by obtaining the current time from an external source, such as from atime server or external clock, or byprompting the user to manually enter the current time.

Thesystem clock is typically implemented as aprogrammable interval timer that periodically interrupts the CPU, which then starts executing a timer interrupt service routine. This routine typically adds one tick to the system clock (a simple counter) and handles other periodic housekeeping tasks (preemption, etc.) before returning to the task the CPU was executing before the interruption.

The following tables illustrate methods for retrieving the system time in variousoperating systems,programming languages, andapplications. Values marked by (*) are system-dependent and may differ across implementations. All dates are given asGregorian orproleptic Gregorian calendar dates.

Theresolution of an implementation's measurement of time does not imply the sameprecision of such measurements. For example, a system might return the current time as a value measured in microseconds, but actually be capable of discerning individual clock ticks with a frequency of only 100 Hz (10 ms).

• Critical and Significant Dates, J. R. Stockton(retrieved 3 December 2015)
• 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 ProjectArchived 2009-02-19 at theWayback Machine (Perl)
• date: Ruby Standard Library Documentation (Ruby)

• Operating system technology
• Computer programming
• Computer real-time clocks

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