.\" $OpenBSD: time2posix.3,v 1.11 2003/05/07 08:52:42 jmc Exp $ .Dd May 24, 1999 .Dt TIME2POSIX 3 .Os .Sh NAME .Nm time2posix , .Nm posix2time .Nd convert seconds since the Epoch .Sh SYNOPSIS .Fd #include .Fd #include .Ft time_t .Fn time2posix "time_t t" .Ft time_t .Fn posix2time "time_t t" .Sh DESCRIPTION IEEE Standard 1003.1 (POSIX) legislates that a .Fa time_t value of 536457599 shall correspond to "Wed Dec 31 23:59:59 UTC 1986." This effectively implies that a POSIX .Fa time_t cannot include leap seconds and, therefore, that the system time must be adjusted as each leap occurs. .Pp If the time package is configured with leap-second support enabled, however, no such adjustment is needed and .Fa time_t values continue to increase over leap events .Po as a true .Sq seconds since... value .Pc . This means that these values will differ from those required by POSIX by the net number of leap seconds inserted since the Epoch. .Pp Typically this is not a problem as the type .Fa time_t is intended to be .Pq mostly opaque. .Fa time_t values should only be obtained from and passed to functions such as .Xr time 3 , .Xr localtime 3 , .Xr mktime 3 , and .Xr difftime 3 . However, POSIX gives an arithmetic expression for directly computing a .Fa time_t value from a given date/time, and the same relationship is assumed by some .Pq usually older applications. Any programs creating/dissecting .Fa time_t values using such a relationship will typically not handle intervals over leap seconds correctly. .Pp The .Fn time2posix and .Fn posix2time functions are provided to address this .Fa time_t mismatch by converting between local .Fa 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. .Pp .Fn time2posix is single-valued. That is, every local .Fa time_t corresponds to a single POSIX .Fa time_t . .Fn 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 .Fa time_t doesn't exist so an adjacent value is returned. Both of these are good indicators of the inferiority of the POSIX representation. .Pp 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. .Bd -ragged -offset indent .ta \w'93/06/30 'u +\w'23:59:59 'u +\w'A+0 'u +\w'X=time2posix(T) 'u 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 .sp .ce [Note: posix2time(B+1) => A+0 or A+1] .Ed .Pp If leap-second support is not enabled, local .Fa time_t and POSIX .Fa time_t are equivalent, and both .Fn time2posix and .Fn posix2time degenerate to the identity function. .Sh SEE ALSO .Xr difftime 3 , .Xr localtime 3 , .Xr mktime 3 , .Xr time 3 .\" @(#)time2posix.3 7.7 .\" This file is in the public domain, so clarified as of .\" 1996-06-05 by Arthur David Olson (arthur_david_olson@nih.gov).