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, how-
     ever, 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).  How-
     ever, 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 typ-
     ically 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 indi-
     cators 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 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 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