1996-06-05 by Arthur David Olson.
.Dd December 15, 2022 .Dt TIME2POSIX 3 .Os .Sh NAME .Nm time2posix , .Nm posix2time .Nd convert seconds since the Epoch .Sh LIBRARY .Lb libc .Sh SYNOPSIS n time.h .Ft time_t .Fn time2posix "time_t t" .Ft time_t .Fn posix2time "time_t t" .Sh DESCRIPTION .St -p1003.1-88 requires the time_t value 536457599 to stand for 1986-12-31 23:59:59 UTC. This effectively implies that POSIX .Vt time_t values cannot include leap seconds and, therefore, that the system time must be adjusted as each leap occurs.
p If the time package is configured with leap-second support enabled, however, no such adjustment is needed and .Vt time_t values continue to increase over leap events (as a true .Dq "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.
p Typically this is not a problem as the type .Vt time_t is intended to be (mostly) opaque \(em .Vt 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, .St -p1003.1-88 gives an arithmetic expression for directly computing a .Vt time_t value from a given date/time, and the same relationship is assumed by some (usually older) applications. Any programs creating/dissecting .Vt time_t values using such a relationship will typically not handle intervals over leap seconds correctly.
p The .Fn time2posix and .Fn posix2time functions are provided to address this .Vt time_t mismatch by converting between local .Vt 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.
p The .Fn time2posix function is single-valued. That is, every local .Vt time_t corresponds to a single POSIX .Vt time_t . The .Fn posix2time function 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 .Vt time_t does not exist so an adjacent value is returned. Both of these are good indicators of the inferiority of the POSIX representation.
p 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. l -column "93/06/30" "23:59:59" "A+0" "X=time2posix(T)" t Sy "DATE TIME T X=time2posix(T) posix2time(X)" t "93/06/30 23:59:59 A+0 B+0 A+0" t "93/06/30 23:59:60 A+1 B+1 A+1 or A+2" t "93/07/01 00:00:00 A+2 B+1 A+1 or A+2" t "93/07/01 00:00:01 A+3 B+2 A+3" .El
p A leap second deletion would look like... l -column "??/06/30" "23:59:58" "A+0" "X=time2posix(T)" t Sy "DATE TIME T X=time2posix(T) posix2time(X)" t "??/06/30 23:59:58 A+0 B+0 A+0" t "??/07/01 00:00:00 A+1 B+2 A+1" t "??/07/01 00:00:01 A+2 B+3 A+2" .El
p .D1 No "[Note: posix2time(B+1) => A+0 or A+1]"
p If leap-second support is not enabled, local .Vt time_t and POSIX .Vt time_t values 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