time_t time2posix(t) time_t t
time_t posix2time(t) time_t t
cc ... -lz
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(2), 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 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 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 positive 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 it's 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
[Note: posix2time(B+1) => A+0 or A+1]
If leap-second 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.