TIME2POSIX(3)

NAME
time2posix, time2posix_z, posix2time, posix2time_z - convert seconds since the Epoch

LIBRARY
libc

SYNOPSIS
#include <time.h>
time_t
time2posix(time_t t);
time_t
time2posix_z(const timezone_t tz, time_t t);
time_t
posix2time(time_t t);
time_t
posix2time_z(const timezone_t tz, time_t t);

DESCRIPTION
p1003.1 requires the time_t value of 536457599 to stand for
    Wed Dec 31 23:59:59 UTC 1986.
This effectively implies that POSIX time_t values 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), localtime_r(3), localtime_rz(3), mktime(3), mktime_z(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(), time2posix_z(), posix2time(), and posix2time_z() 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() and time2posix_z() are single-valued. That is, every local time_t corresponds to a single POSIX time_t. posix2time() and posix2time() are 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 “z” variants of the two functions behave exactly like their counterparts, but they operate in the given tz argument which was previously allocated using tzalloc(3) and are re-entrant.

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 values are equivalent, and both time2posix() and posix2time() degenerate to the identity function.

SEE ALSO
difftime(3), localtime(3), localtime_r(3), localtime_rz(3), mktime(3), mktime_z(3), time(3), tzalloc(3)