time2posix(3)              Library Functions Manual              time2posix(3)

NAME
       time2posix, posix2time - convert seconds since the Epoch

SYNOPSIS
       #include <time.h>

       time_t time2posix(time_t t);

       time_t posix2time(time_t t);

       cc ... -ltz

DESCRIPTION
       IEEE Standard 1003.1 (POSIX) requires the time_t value 536457599 to
       stand for 1986-12-31 23:59:59 UTC.  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(2), localtime(3), mktime(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 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 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), mktime(3), time(2)

Time Zone Database                                               time2posix(3)