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

NAME
     time2posix, time2posix_z, posix2time, posix2time_z, — convert seconds
     since the Epoch

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     time_t
     time2posix(time_t t);

     time_t
     time2posix_z(const timezone_t tz, time_t t);

     time_t
     posix2time(time_t t);

     time_t
     posix2time_z(const timezone_t tz, time_t t);

DESCRIPTION
     IEEE Std 1003.1 (“POSIX.1”) legislates that a time_t value of 536457599
     shall correspond to
	   Wed Dec 31 23:59:59 UTC 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), localtime_r(3),
     localtime_rz(3), mktime(3), mktime_z(3), and difftime(3).	However, POSIX
     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 typically not handle intervals over leap seconds
     correctly.

     The time2posix(), time2posix_z(), posix2time(), and posix2time_z() func‐
     tions 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 appli‐
     cations, or when communicating with POSIX-compliant systems.

     time2posix() and time2posix_z() are single-valued.	 That is, every local
     time_t corresponds to a single POSIX time_t.  posix2time() and
     posix2time() are less well-behaved: for a positive leap second hit the
     result is not unique, and for a negative leap second hit the correspond‐
     ing POSIX time_t doesn't exist so an adjacent value is returned.  Both of
     these are good indicators of the inferiority of the POSIX representation.

     The “z” variants of the two functions behave exactly like their counter‐
     parts, but they operate in the given tz argument which was previously
     allocated using tzalloc(3) and are re-entrant.

     The following table summarizes the relationship between a 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), localtime_r(3), localtime_rz(3), mktime(3),
     mktime_z(3), time(3), tzalloc(3)

BSD			       December 4, 2010				   BSD
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