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+.\" $FreeBSD$
+.\"
+.Dd September 11, 2005
+.Dt TIME2POSIX 3
+.Os
+.Sh NAME
+.Nm time2posix ,
+.Nm posix2time
+.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 posix2time "time_t t"
+.Sh DESCRIPTION
+.St -p1003.1-88
+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.
+.Pp
+If the time package is configured with leap-second support
+enabled,
+however,
+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.
+.Pp
+Typically this is not a problem as the type time_t is intended
+to be
+(mostly)
+opaque\(emtime_t values should only be obtained-from and
+passed-to functions such as
+.Xr time 3 ,
+.Xr localtime 3 ,
+.Xr mktime 3
+and
+.Xr difftime 3 .
+However,
+.St -p1003.1-88
+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.
+.Pp
+The
+.Fn time2posix
+and
+.Fn 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.
+.Pp
+The
+.Fn time2posix
+function is single-valued.
+That is,
+every local time_t
+corresponds to a single POSIX time_t.
+The
+.Fn 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 indicators of the inferiority of the
+POSIX representation.
+.Pp
+The following table summarizes the relationship between 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)"
+.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)"
+.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
+.Pp
+.D1 No "[Note: posix2time(B+1) => A+0 or A+1]"
+.Pp
+If leap-second support is not enabled,
+local time_t's and
+POSIX time_t's are equivalent,
+and both
+.Fn time2posix
+and
+.Fn posix2time
+degenerate to the identity function.
+.Sh "SEE ALSO"
+.Xr difftime 3 ,
+.Xr localtime 3 ,
+.Xr mktime 3 ,
+.Xr time 3