[LEAPSECS] Civil timekeeping before 1 January 1972

Steve Allen sla at ucolick.org
Sat Mar 7 15:01:12 EST 2015

On Sat 2015-03-07T14:14:07 -0500, Brooks Harris hath writ:
> It is typically warned that date and time before 1972 cannot be
> accurately represented with NTP or POSIX, for examples.

I would say that for PTP
* all seconds are always SI seconds
* seconds after 1972-01-01 correspond to (TAI - 10)
* seconds before 1972-01-01 do not align with civil time
* in particular, 1970-01-01 in PTP does not correspond to
  any event in any time scale which was then in use,
  but that does not matter
For PTP it is the uniformity going forward that is the goal.

I would say that the intent NTP and POSIX is to correspond to civil
time in contemporary use.  Therefore, for dates before 1972-01-01
NTP and POSIX are counting seconds of UT.

For dates after 1972-01-01 I would say that NTP and POSIX are both
confused about whether they are counting SI seconds or UT days.
Therein, of course, lies the basis of this LEAPSECS list.

I see it as inevitable that the confusion must end, therefore NTP and
POSIX will eventually be unambiguous that they are counting SI
seconds.  That means that, eventually, any general purpose time scale
intended to correspond to civil time will be a piecewise continuous
time scale.  Such general purpose time scales will have a date at
which they switch from counting UT days to counting SI seconds.

Deciding on that date, and how it will be implemented and understood,
is what we hope the ITU-R will accomplish at WRC-15.

> For date-time before 1972 you've got to switch to some
> other timescale depending on the purpose at hand.

Exactly so.  Before 1972 civil time was not SI seconds.

Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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