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    1 TIME2POSIX(3)              Library Functions Manual              TIME2POSIX(3)
    2 
    3 NAME
    4        time2posix, posix2time - convert seconds since the Epoch
    5 
    6 SYNOPSIS
    7        #include <time.h>
    8 
    9        time_t time2posix(time_t t);
   10 
   11        time_t posix2time(time_t t);
   12 
   13        cc ... -ltz
   14 
   15 DESCRIPTION
   16        IEEE Standard 1003.1 (POSIX) requires the time_t value 536457599 to
   17        stand for 1986-12-31 23:59:59 UTC.  This effectively implies that POSIX
   18        time_t values cannot include leap seconds and, therefore, that the
   19        system time must be adjusted as each leap occurs.
   20 
   21        If the time package is configured with leap-second support enabled,
   22        however, no such adjustment is needed and time_t values continue to
   23        increase over leap events (as a true "seconds since..."  value).  This
   24        means that these values will differ from those required by POSIX by the
   25        net number of leap seconds inserted since the Epoch.
   26 
   27        Typically this is not a problem as the type time_t is intended to be
   28        (mostly) opaque - time_t values should only be obtained-from and
   29        passed-to functions such as time(2), localtime(3), mktime(3), and
   30        difftime(3).  However, POSIX gives an arithmetic expression for
   31        directly computing a time_t value from a given date/time, and the same
   32        relationship is assumed by some (usually older) applications.  Any
   33        programs creating/dissecting time_t's using such a relationship will
   34        typically not handle intervals over leap seconds correctly.
   35 
   36        The time2posix and posix2time functions are provided to address this
   37        time_t mismatch by converting between local time_t values and their
   38        POSIX equivalents.  This is done by accounting for the number of time-
   39        base changes that would have taken place on a POSIX system as leap
   40        seconds were inserted or deleted.  These converted values can then be
   41        used in lieu of correcting the older applications, or when
   42        communicating with POSIX-compliant systems.
   43 
   44        Time2posix is single-valued.  That is, every local time_t corresponds
   45        to a single POSIX time_t.  Posix2time is less well-behaved: for a
   46        positive leap second hit the result is not unique, and for a negative
   47        leap second hit the corresponding POSIX time_t doesn't exist so an
   48        adjacent value is returned.  Both of these are good indicators of the
   49        inferiority of the POSIX representation.
   50 
   51        The following table summarizes the relationship between a time T and
   52        it's conversion to, and back from, the POSIX representation over the
   53        leap second inserted at the end of June, 1993.
   54        DATE     TIME     T   X=time2posix(T) posix2time(X)
   55        93/06/30 23:59:59 A+0 B+0             A+0
   56        93/06/30 23:59:60 A+1 B+1             A+1 or A+2
   57        93/07/01 00:00:00 A+2 B+1             A+1 or A+2
   58        93/07/01 00:00:01 A+3 B+2             A+3
   59 
   60        A leap second deletion would look like...
   61 
   62        DATE     TIME     T   X=time2posix(T) posix2time(X)
   63        ??/06/30 23:59:58 A+0 B+0             A+0
   64        ??/07/01 00:00:00 A+1 B+2             A+1
   65        ??/07/01 00:00:01 A+2 B+3             A+2
   66 
   67                             [Note: posix2time(B+1) => A+0 or A+1]
   68 
   69        If leap-second support is not enabled, local time_t's and POSIX
   70        time_t's are equivalent, and both time2posix and posix2time degenerate
   71        to the identity function.
   72 
   73 SEE ALSO
   74        difftime(3), localtime(3), mktime(3), time(2)
   75 
   76                                                                  TIME2POSIX(3)