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TIME2POSIX(3)            BSD Library Functions Manual            TIME2POSIX(3)

NAME
     time2posix, posix2time -- convert seconds since the Epoch

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     time_t
     time2posix(time_t t);

     time_t
     posix2time(time_t t);

DESCRIPTION
     IEEE Std 1003.1-1988 (``POSIX.1'') 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, IEEE Std 1003.1-1988 (``POSIX.1'') 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 typi-cally typically
     cally 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 does not exist so an
     adjacent value is returned.  Both of these are good indicators of the inferiority of the POSIX repre-sentation. representation.
     sentation.

     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)

BSD                           September 11, 2005                           BSD