time2posix man page on FreeBSD

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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, 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 pro‐
     grams creating/dissecting time_t's using such a relationship will typi‐
     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 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
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