| TIME2POSIX(3) | Library Functions Manual | TIME2POSIX(3) | 
time2posix, time2posix_z,
  posix2time, posix2time_z
  —
#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);
536457599 to
  stand for
Wed Dec 31 23:59:59 UTC
  1986.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's 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 | 
If leap-second support is not enabled, local
    time_t's and POSIX time_t's are
    equivalent, and both time2posix() and
    posix2time() degenerate to the identity
  function.
| October 6, 2014 | NetBSD 9.4 |