TIME2POSIX(3)

NAME
time2posix, posix2time - convert seconds since the Epoch

LIBRARY
libc

SYNOPSIS
#include <time.h>
time_t
time2posix(const time_t *t);
time_t
posix2time(const time_t *t);

DESCRIPTION
p1003.1-88 legislates that a time_t value of 536457599 shall correspond to "Wed Dec 31 23:59:59 GMT 1986." This effectively implies that POSIX time_t's 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, p1003.1-88 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() 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.

The time2posix() function is single-valued. That is, every local time_t corresponds to a single POSIX time_t. The 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 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 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's and POSIX time_t's are equivalent, and both time2posix() and posix2time() degenerate to the identity function.

SEE ALSO
difftime(3), localtime(3), mktime(3), time(3)