-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_ts 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, 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) opaquetime_t values should only be obtained-from and passed-to functions such as time(3), localtime(3), mktime(3) and difftime(3). However, -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_ts 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.
The time2posix function is single-valued. That is, every local time_t corresponds to a single POSIX time_t. The 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.
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.