IEEE Standard 1003.1 (POSIX) requires the `time_t` value 536457599 to stand for
1986-12-31 23:59:59 UTC. This effectively implies that POSIX `time_t` values
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)
opaque---`time_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` values 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` 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 its
conversion to, and back from, the POSIX representation over the leap second
inserted at the end of June, 1993.
| 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...
| ??/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` and POSIX `time_t` values
are equivalent, and both `time2posix` and `posix2time` degenerate to the
identity function.