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RocketLink!--> Man page versions: OpenBSD FreeBSD Others

TIME2POSIX(3)             OpenBSD Programmer's Manual            TIME2POSIX(3)

     time2posix, posix2time - convert seconds since the Epoch

     #include <sys/types.h>
     #include <time.h>

     time2posix(time_t t);

     posix2time(time_t t);

     IEEE Standard 1003.1 (POSIX) legislates that a time_t value of 536457599
     shall correspond to "Wed Dec 31 23:59:59 UTC 1986."  This effectively im-
     plies that a POSIX time_t 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).  However, POSIX gives an arithmetic expression for directly
     computing a time_t value from a given date/time, and the same relation-
     ship is assumed by some (usually older) applications.  Any programs cre-
     ating/dissecting time_t values using such a relationship will typically
     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.

     time2posix() is single-valued.  That is, every local time_t corresponds
     to a single POSIX time_t. posix2time() is less well-behaved: for a posi-
     tive leap second hit the result is not unique, and for a negative leap
     second hit the corresponding 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 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 and POSIX time_t are
     equivalent, and both time2posix() and posix2time() degenerate to the
     identity function.

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

OpenBSD 2.6                      May 24, 1999                                2

Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.

(Corrections, notes, and links courtesy of RocketAware.com)

[Detailed Topics]
FreeBSD Sources for time2posix(3) functions
OpenBSD sources for time2posix(3)

[Overview Topics]

Up to: Calendar and Time of Day - Calendar and Time of Day (conversions, manipulations, etc)

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