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

NAME
     time2posix, posix2time -- convert seconds since the Epoch

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     ##include <>

     timet
     time2posix(timet t);

     timet
     posix2time(timet t);

DESCRIPTION
     IE Std 1003.1-1988 (``POSIX.1'') legislates that a timet value of
     536457599 shall correspond to "Wed Dec 31 23:59:59 GMT 1986."  This
     effectively implies that POSIX timet'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, how-
     ever, no such adjustment is needed and timet 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 timet is intended to be
     (mostly) opaque--timet values should only be obtained-from and passed-to
     functions such as time(3), localtime(3), mktime(3) and difftime(3).  How-
     ever, IE Std 1003.1-1988 (``POSIX.1'') gives an arithmetic expression
     for directly computing a timet value from a given date/time, and the
     same relationship is assumed by some (usually older) applications.  Any
     programs creating/dissecting timet's using such a relationship will typ-
     ically not handle intervals over leap seconds correctly.

     The time2posix() and posix2time() functions are provided to address this
     timet mismatch by converting between local timet 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 timet
     corresponds to a single POSIX timet.  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 timet doesn't
     exist so an adjacent value is returned.  Both of these are good indica-
     tors of the inferiority of the POSIX representation.

     The following table summarizes the relationship between timet and its
     conversion to, and back from, the POSIX representation over the leap sec-
     ond 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 timet's and POSIX timet's
     are equivalent, and both time2posix() and posix2time() degenerate to the
     identity function.

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

BSD                               May 1, 1996                              BSD