p1003.188
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 leapsecond 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.
Typically this is not a problem as the type time_t is intended
to be
(mostly)
opaque—time_t values should only be obtainedfrom and
passedto functions such as
time(3),
localtime(3),
mktime(3)
and
difftime(3).
However,
p1003.188
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
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 timebase 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 POSIXcompliant systems.
The
time2posix
function is singlevalued.
That is,
every local time_t
corresponds to a single POSIX time_t.
The
posix2time
function is less wellbehaved:
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.
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.
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




If leapsecond 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.