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Many computer systems measure time and date using Unix time, an international standard for digital timekeeping. Unix time is defined as the number of seconds elapsed since 00:00:00 UTC on 1 January 1970 (an arbitrarily chosen time based on the creation of the first Unix system), which has been dubbed the Unix epoch. [6]
Unix time [a] is a date and time representation widely used in computing. It measures time by the number of non-leap seconds that have elapsed since 00:00:00 UTC on 1 January 1970, the Unix epoch. For example, at midnight on 1 January 2010, Unix time was 1262304000. Unix time originated as the system time of Unix operating systems.
The C date and time functions are a group of functions in the standard library of the C programming language implementing date and time manipulation operations. [1] They provide support for time acquisition, conversion between date formats, and formatted output to strings.
Software timekeeping systems vary widely in the resolution of time measurement; some systems may use time units as large as a day, while others may use nanoseconds.For example, for an epoch date of midnight UTC (00:00) on 1 January 1900, and a time unit of a second, the time of the midnight (24:00) between 1 January 1900 and 2 January 1900 is represented by the number 86400, the number of ...
System time can be converted into calendar time, which is a form more suitable for human comprehension. For example, the Unix system time 1 000 000 000 seconds since the beginning of the epoch translates into the calendar time 9 September 2001 01:46:40 UT.
On 5 January 1975, the 12-bit field that had been used for dates in the TOPS-10 operating system for DEC PDP-10 computers overflowed, in a bug known as "DATE75". The field value was calculated by taking the number of years since 1964, multiplying by 12, adding the number of months since January, multiplying by 31, and adding the number of days since the start of the month; putting 2 12 − 1 ...
The term "real time" in this context refers to elapsed wall-clock time, like using a stop watch. The total CPU time (user time + sys time) may be more or less than that value. Because a program may spend some time waiting and not executing at all (whether in user mode or system mode) the real time may be greater than the total CPU time.
The Unix command time prints CPU time and elapsed real time for the execution of a Unix command (or pipeline). Note that many command-line shells have their own implementation of this command. To run the Unix program time , we provide its full path, /usr/bin/time :