.\"	$NetBSD: time2posix.3,v 1.23 2022/12/11 17:57:23 christos Exp $
.\" @(#)time2posix.3	7.7
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson.
.Dd October 6, 2014
.Dt TIME2POSIX 3
.Os
.Sh NAME
.Nm time2posix ,
.Nm time2posix_z ,
.Nm posix2time ,
.Nm posix2time_z
.Nd convert seconds since the Epoch
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Ft time_t
.Fn time2posix "time_t t"
.Ft time_t
.Fn time2posix_z "const timezone_t tz" "time_t t"
.Ft time_t
.Fn posix2time "time_t t"
.Ft time_t
.Fn posix2time_z "const timezone_t tz" "time_t t"
.Sh DESCRIPTION
.St -p1003.1
requires the
.Ft time_t
value of
.Dv 536457599
to stand for
.Dl Wed Dec 31 23:59:59 UTC 1986 .
This effectively implies that POSIX
.Ft time_t
values cannot include leap seconds and, therefore,
that the system time must be adjusted as each leap occurs.
.Pp
If the time package is configured with leap-second support
enabled, however, no such adjustment is needed and
.Va time_t
values continue to increase over leap events
(as a true
.Dq "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.
.Pp
Typically this is not a problem as the type
.Va time_t
is intended to be (mostly)
opaque \(em
.Va time_t
values should only be obtained-from and
passed-to functions such as
.Xr time 3 ,
.Xr localtime 3 ,
.Xr localtime_r 3 ,
.Xr localtime_rz 3 ,
.Xr mktime 3 ,
.Xr mktime_z 3 ,
and
.Xr difftime 3 .
However, POSIX gives an arithmetic expression for directly computing a
.Va time_t
value from a given date/time, and the same relationship is assumed by
some (usually older) applications.
Any programs creating/dissecting
.Va time_t
values using such a relationship will typically not handle intervals over
leap seconds correctly.
.Pp
The
.Fn time2posix ,
.Fn time2posix_z ,
.Fn posix2time ,
and
.Fn posix2time_z
functions are provided to address this
.Va time_t
mismatch by converting between local
.Va 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.
.Pp
.Fn time2posix
and
.Fn time2posix_z
are single-valued.
That is, every local
.Va time_t
corresponds to a single POSIX
.Va time_t .
.Fn posix2time
and
.Fn posix2time
are less well-behaved: for a positive leap second hit the result is not
unique, and for a negative leap second hit the corresponding POSIX
.Va time_t
doesn't exist so an adjacent value is returned.
Both of these are good indicators of the inferiority of the POSIX
representation.
.Pp
The
.Dq z
variants of the two functions behave exactly like their counterparts,
but they operate in the given
.Fa tz
argument which was previously allocated using
.Xr tzalloc 3
and are re-entrant.
.Pp
The following table summarizes the relationship between a
.Va time_t
and its conversion to, and back from, the POSIX representation over
the leap second inserted at the end of June, 1993.
.Bl -column "93/06/30" "23:59:59" "A+0" "X=time2posix(T)" "posix2time(X)" -offset indent
.It Sy DATE	TIME	T	X=time2posix(T)	posix2time(X)
.It 93/06/30	23:59:59	A+0	B+0	A+0
.It 93/06/30	23:59:60	A+1	B+1	A+1 or A+2
.It 93/07/01	00:00:00	A+2	B+1	A+1 or A+2
.It 93/07/01	00:00:01	A+3	B+2	A+3
.El
.Pp
A leap second deletion would look like...
.Bl -column "??/06/30" "23:59:58" "A+0" "X=time2posix(T)" "posix2time(X)" -offset indent
.It Sy DATE	TIME	T	X=time2posix(T)	posix2time(X)
.It ??/06/30	23:59:58	A+0	B+0	A+0
.It ??/07/01	00:00:00	A+1	B+2	A+1
.It ??/07/01	00:00:01	A+2	B+3	A+2
.El
[Note: posix2time(B+1) => A+0 or A+1]
.Pp
If leap-second support is not enabled, local
.Va time_t
and POSIX
.Va time_t
values are equivalent, and both
.Fn time2posix
and
.Fn posix2time
degenerate to the identity function.
.Sh SEE ALSO
.Xr difftime 3 ,
.Xr localtime 3 ,
.Xr localtime_r 3 ,
.Xr localtime_rz 3 ,
.Xr mktime 3 ,
.Xr mktime_z 3 ,
.Xr time 3 ,
.Xr tzalloc 3