Hello Brooks;
On 2016-12-28 12:33, Brooks Harris wrote:
On 2016-12-26 08:28 PM, Tony Finch wrote:
Brooks Harris<[email protected]> wrote:
The time_t 1970 epoch is fixed with respect to internal POSIX calculations,
but it "slips" a second with respect to UTC with each (positive) Leap Second
introduction because "23:59:60" goes missing.
"Slip" makes it sound like a retrospective change in the relationship
between UTC and time_t, with the scales sliding against each other, but it
doesn't change - there's a fixed relationship between UTC second labels
and time_t second labels, which remains the same however many leap seconds
there are. In fact time_t is defined in terms of that relationship, and
"seconds since the epoch" is just a simplified gloss.
Hi Tony,
"Slip" might not be the best term, but the effective alignment between
time_t and UTC changes with each Leap Second.
POSIX "the epoch" origin, time_t = zero, is said to be coincident with
"1970-01-01 00:00:00 UTC".
The YMDhms count progression across the first Leap Second
(1972-06-30T23:59:60 (UTC)) as yielded by POSIX gmtime() is expected
to be
time_t gmtime() UTC
78796799 = 1972-06-30 23:59:59 = 1972-06-30T23:59:59 (UTC)
78796800 = 1972-07-01 00:00:00 = 1972-06-30T23:59:60 (UTC) << Leap Second
78796800 = 1972-07-01 00:00:00 = 1972-07-01T00:00:00 (UTC) << time_t reset
78796801 = 1972-07-01 00:00:01 = 1972-07-01T00:00:01 (UTC)
time_t must be reset after the Leap Second to maintain the alignment
of the POSIX and UTC YMDhms representations. In effect the time_t
origin has become coincident with "1972-01-01 00:00:00 UTC plus one
Leap Second", or "1972-01-01 00:00:01 UTC". As David Mills says "...
In effect, a new timescale is reestablished after each new leap second."
The NTP Timescale and Leap Seconds
3. How NTP and POSIX Reckon with Leap Seconds
https://www.eecis.udel.edu/~mills/leap.html
[ As discussed on the list many times, POSIX's use of the term "UTC"
in describing "the epoch" as "1970-01-01 00:00:00 UTC" is troublesome
because it lies before 1972-01-01T00:00:00 (UTC) in the "rubber band
era", the development period of TAI and UTC when the TAI-UTC offset
was not integral seconds. The name of the timescale and the TAI-UTC
values during that span is somewhat controversial. In any event, as a
practical matter, the origin of POSIX "the epoch" is usually taken to
mean 63072000 seconds
That should be 63072010 seconds (730 days x 86400 plus 10 seconds)
-Stephen
before 1972-01-01 00:00:10 (TAI) = 1972-01-01T00:00:00 (UTC) ].
-Brooks
Tony.
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