[LEAPSECS] Leap second relationship to ISO 8601

[Tony Finch]

Brooks Harris <brooks at edlmax.com> wrote:
>
> > > Most explainations, and the spec itself, say that NTP does not account
> > > for Leap Seconds, and it does not, *explicitly*. Note, however, that
> > > 2272060800 secs / 86400 = 26297 days *exactly*. There were 10 Leap
> > > Seconds in effect at 1972-01-01T00:00:00Z (UTC), so there must be 10
> > > Leap Seconds in effect at the NTP "prime epoch of era zero", "0 h 1
> > > January 1900 UTC".
> > That reasoning is faulty.
> I hope not :-\
>
> > Figure 4 of RFC 5905 also says that the
> > NTP timestamp for 1999-12-31 is 3155587200, and
> > (3155587200-2272060800)/86400 is 10226 days exactly.
> 1999-12-31 is after 1972, and 32 Leap Seconds are in effect - 10 initial at
> 1972 plus 22 decreed inserts.

OK, so apply that same reasoning to dates before 1972, when UTC had
subsecond leaps and rate changes. Why not say that during this period the
NTP timescale (had it existed) would also have had subsecond leaps and
rate changes, communicated separately from the timestamps like leapseconds
are now?

A fixed 10s offset is obviously wrong because there is a fixed offset
between POSIX and NTP timestamps (except during leap seconds), so there is
a POSIX-like fixed mapping from wall clock time to NTP timestamps. (By
"wall clock time" I mean UT of whichever variety was in use at that time.)
So at the point in time where TAI was established, when it coincided with
UT, the NTP leap second count has to be 0.

Tony.
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