System time

In <a href="/facts/Computer_science/lN3pEyPz">computer science</a> and <a href="/facts/Computer_programming/QmSsFnu0">computer programming</a>, system time represents a computer system's notion of the passage of time. In this sense, time also includes the passing of <a href="/facts/Calendar_date/3Ive9Wwb">days on the calendar</a>.
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 <a href="/facts/Epoch_(computing)/dA2Gyueu">epoch</a>. For example, <a href="/facts/Unix/0pNL2seI">Unix</a> and <a href="/facts/POSIX/mQkSt234">POSIX</a>-compliant systems encode system time ("<a href="/facts/Unix_time/d8Et1sa2">Unix time</a>") as the number of seconds elapsed since the start of the <a href="/facts/Unix_epoch/d8Et1sa2">Unix epoch</a> at 1 January 1970 00:00:00 <a href="/facts/Universal_Time/l1FIFGKn">UT</a>, with exceptions for <a href="/facts/Leap_second/HQQ0X0hn">leap seconds</a>. Systems that implement the 32-bit and 64-bit versions of the <a href="/facts/Windows_API/RRw4RtTu">Windows API</a>, such as <a href="/facts/Windows_9x/f1zwlghw">Windows 9x</a> and <a href="/facts/Windows_NT/EtccWvxQ">Windows NT</a>, 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 <a href="/facts/Proleptic_Gregorian_calendar/168ukkHr">proleptic Gregorian calendar</a>.
System time can be converted into <a href="/facts/Calendar_date/3Ive9Wwb">calendar time</a>, which is a form more suitable for human comprehension. For example, the <a href="/facts/Unix_time/d8Et1sa2">Unix system time</a> 1000000000 seconds since the beginning of the epoch translates into the calendar time 9 September 2001 01:46:40 UT. Library <a href="/facts/Subroutine/9vFJrg0A">subroutines</a> that handle such conversions may also deal with adjustments for <a href="/facts/Time_zone/x5JzdlX8">time zones</a>, <a href="/facts/Daylight_saving_time/bEWvB1Rt">daylight saving time</a> (DST), leap seconds, and the user's <a href="/facts/Locale_(computer_software)/Hwbtpj4e">locale</a> 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 impending <a href="/facts/Year_2038_problem/dsvSnTgh">Year 2038 problem</a>. These time values will overflow ("run out of bits") after the end of their system time epoch, leading to <a href="/facts/Time_formatting_and_storage_bugs/1K5tbwIa">software and hardware errors</a>. These systems will require some form of remediation, similar to efforts required to solve the earlier <a href="/facts/Year_2000_problem/GQwkI92d">Year 2000 problem</a>. This will also be a potentially much larger problem for existing <a href="/facts/File_format/qrXbwJaH">data file formats</a> that contain system timestamps stored as 32-bit values.

System time open-in-new

System time