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TIME2POSIX(3) FreeBSD Library Functions Manual TIME2POSIX(3) NAME time2posix, posix2time - convert seconds since the Epoch SYNOPSIS #include <sys/types.h> #include <time.h> time_t time2posix(time_t t); time_t posix2time(time_t t); DESCRIPTION IEEE Std 1003.1 ("POSIX.1") legislates that a time_t value of 536457599 shall correspond to "Wed Dec 31 23:59:59 UTC 1986". This effectively implies that a POSIX time_t cannot include leap seconds and, therefore, that the system time must be adjusted as each leap occurs. If the time package is configured with leap-second support enabled, however, no such adjustment is needed and time_t values continue to increase over leap events (as a true `seconds since...' value). This means that these values will differ from those required by POSIX by the net number of leap seconds inserted since the Epoch. Typically this is not a problem as the type time_t is intended to be (mostly) opaque. time_t values should only be obtained from and passed to functions such as time(3), localtime(3), mktime(3), and difftime(3). However, POSIX gives an arithmetic expression for directly computing a time_t value from a given date/time, and the same relationship is assumed by some (usually older) applications. Any programs creating/dissecting time_t values using such a relationship will typically not handle intervals over leap seconds correctly. The time2posix() and posix2time() functions are provided to address this time_t mismatch by converting between local time_t values and their POSIX equivalents. This is done by accounting for the number of time-base changes that would have taken place on a POSIX system as leap seconds were inserted or deleted. These converted values can then be used in lieu of correcting the older applications, or when communicating with POSIX-compliant systems. time2posix() is single-valued. That is, every local time_t corresponds to a single POSIX time_t. posix2time() is less well-behaved: for a positive leap second hit the result is not unique, and for a negative leap second hit the corresponding POSIX time_t doesn't exist so an adjacent value is returned. Both of these are good indicators of the inferiority of the POSIX representation. The following table summarizes the relationship between a time T and its conversion to, and back from, the POSIX representation over the leap second inserted at the end of June, 1993. DATE TIME T X=time2posix(T) posix2time(X) 93/06/30 23:59:59 A+0 B+0 A+0 93/06/30 23:59:60 A+1 B+1 A+1 or A+2 93/07/01 00:00:00 A+2 B+1 A+1 or A+2 93/07/01 00:00:01 A+3 B+2 A+3 A leap second deletion would look like... DATE TIME T X=time2posix(T) posix2time(X) ??/06/30 23:59:58 A+0 B+0 A+0 ??/07/01 00:00:00 A+1 B+2 A+1 ??/07/01 00:00:01 A+2 B+3 A+2 [Note: posix2time(B+1) => A+0 or A+1] If leap-second support is not enabled, local time_t and POSIX time_t are equivalent, and both time2posix() and posix2time() degenerate to the identity function. SEE ALSO difftime(3), localtime(3), mktime(3), time(3) FreeBSD 14.1-RELEASE-p8 February 23, 2015 FreeBSD 14.1-RELEASE-p8