[LEAPSECS] POSIX Time

[M. Warner Losh]

In message: <p06240804c6f6cee17012@[192.168.1.212]>
            Joe Gwinn <joegwinn at comcast.net> writes:

: >  >Another common source of confusion is that the POSIX Epoch is an instant

: >>defined in UTC terms,

: >

: >... and occurring at a time for which the present form of UTC is

: >undefined.  I don't think anyone actually attempts to apply the POSIX

: >time_t definition to pre-1972 (pre-leap-seconds) UTC.  De facto, Unix

: >timestamps of any significant age cannot be precisely related to UTC

: >(or TAI or any other precision time scale).  Historical time_t values

: >can at best only be interpreted as a transformation of vague UT, unusable

: >for sub-second absolute timing.  (Actually, you won't often see pre-1990

: >timestamps that are accurate to the minute, let alone precise enough to

: >distinguish between flavours of UT.)

: 

: All kinda true, but for the intended use of POSIX Time, the errors 

: are not significant bu the relevant users.  For instance, the UTC 

: definition of the POSIX Epoch (originally defined in terms of GMT) is 

: off by about 80 microseconds (if memory serves).


Yes, time_t is usually talked about in terms of a fake UTC that never
existed: one where seconds were uniform and there were no steps in the
time scale.  Neither one of these were true.  About 2 seconds of UTC /
TAI divergence accumulated during 1970 and 1972.  The delta between
UTC and TAI was fixed at 10 at the start of 1972.  It on the order of
80ms shy of 8s on Jan 1, 1970, but I haven't done the math lately.

Time_t totally lacks the ability to accurately portray the rubber
seconds that pre-dated 1972.  And that bit of abnormality is usually
ignored for the sake of simplicity.

So in many ways UTC and time_t are only superficially similar things.
time_t is a half-assed attempt to do the right thing for time.  It
generally works for most people most of the time, but is wrong where
it doesn't match reality.

Warner