1 TIME2POSIX(3) TIME2POSIX(3)
4 time2posix, posix2time - convert seconds since the Epoch
19 IEEE Standard 1003.1 (POSIX) legislates that a time_t value of
20 536457599 shall correspond to "Wed Dec 31 23:59:59 UTC 1986." This
21 effectively implies that POSIX time_t's cannot include leap seconds
22 and, therefore, that the system time must be adjusted as each leap
25 If the time package is configured with leap-second support enabled,
26 however, no such adjustment is needed and time_t values continue to
27 increase over leap events (as a true `seconds since...' value). This
28 means that these values will differ from those required by POSIX by the
29 net number of leap seconds inserted since the Epoch.
31 Typically this is not a problem as the type time_t is intended to be
32 (mostly) opaque--time_t values should only be obtained-from and passed-
33 to functions such as time(2), localtime(3), mktime(3), and difftime(3).
34 However, POSIX gives an arithmetic expression for directly computing a
35 time_t value from a given date/time, and the same relationship is
36 assumed by some (usually older) applications. Any programs
37 creating/dissecting time_t's using such a relationship will typically
38 not handle intervals over leap seconds correctly.
40 The time2posix and posix2time functions are provided to address this
41 time_t mismatch by converting between local time_t values and their
42 POSIX equivalents. This is done by accounting for the number of time-
43 base changes that would have taken place on a POSIX system as leap
44 seconds were inserted or deleted. These converted values can then be
45 used in lieu of correcting the older applications, or when
46 communicating with POSIX-compliant systems.
48 Time2posix is single-valued. That is, every local time_t corresponds
49 to a single POSIX time_t. Posix2time is less well-behaved: for a
50 positive leap second hit the result is not unique, and for a negative
51 leap second hit the corresponding POSIX time_t doesn't exist so an
52 adjacent value is returned. Both of these are good indicators of the
53 inferiority of the POSIX representation.
55 The following table summarizes the relationship between a time T and
56 it's conversion to, and back from, the POSIX representation over the
57 leap second inserted at the end of June, 1993.
58 DATE TIME T X=time2posix(T) posix2time(X)
59 93/06/30 23:59:59 A+0 B+0 A+0
60 93/06/30 23:59:60 A+1 B+1 A+1 or A+2
61 93/07/01 00:00:00 A+2 B+1 A+1 or A+2
62 93/07/01 00:00:01 A+3 B+2 A+3
64 A leap second deletion would look like...
66 DATE TIME T X=time2posix(T) posix2time(X)
67 ??/06/30 23:59:58 A+0 B+0 A+0
68 ??/07/01 00:00:00 A+1 B+2 A+1
69 ??/07/01 00:00:01 A+2 B+3 A+2
71 [Note: posix2time(B+1) => A+0 or A+1]
73 If leap-second support is not enabled, local time_t's and POSIX
74 time_t's are equivalent, and both time2posix and posix2time degenerate
75 to the identity function.
78 difftime(3), localtime(3), mktime(3), time(2)