[LEAPSECS] Civil timekeeping before 1 January 1972

[Brooks Harris]

Hi Steve,

On 2015-03-07 03:01 PM, Steve Allen wrote:
> 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.
Keep in mind PTP is for ".. Networked Measurement and Control Systems", 
so its first objective is for things like process control, hence its 
primary timescale is "TAI-like". Civil time, or UTC, is a secondary 
concideration, but it has mechanisms to communicate UTC 
(currentUtcOffset, leap59, leap61). It does not specify the UTC rules by 
which these are set, relying on the UTC background specifications for that.

leap59 and leap61 are Leap Second announce signals, set 12 hours prior 
to the insert. There has been discussion about when the official 
announcements and expiration should be announced. ITU Rec 460 says 
"...at least eight weeks in advance". PTP can't do that, a point to keep 
in mind.

> 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.
First let me compliment you on all your excellent work and research at 
http://www.ucolick.org/~sla/leapsecs/. This is an invaluable resource 
and I have learned much from it. Thank you.

This paragraph in your email had me scratching my head a little. So, 
first, off to your time pages to learn more carefully about UT. I think 
I see what you mean generally.

My view is the NTP and POSIX are abstract timescales constructed for 
convenient "civil" timekeeping calculation, albeit with their Leap 
Second flaws. I think I would suggest your statement would be better as 
" .. for dates before 1972-01-01 NTP and POSIX are counting seconds on a 
proleptic scale that extrapolates the SI Second prior to 1972-01-01.".

This seems clear epecially in NTP, where RFC 5905 states "It should be 
noted that strictly speaking, UTC did not exist prior to 1 January 1972, 
but it is convenient to assume it has existed for all eternity, even if 
all knowledge of historic leap seconds has been lost."

It seems to me NTP and POSIX as well as other timescales concerned with 
"civil time", are essentially disconnected from "reality", expressing 
"idealized" measurement scales. UTC is an idealized scale expressing an 
approximation of reality derived from UT1, and UT1 will probably always 
undergo some refinement and the sciences advance. I think none of the 
"civil" timescales are counting in UT - they are measured in SI Seconds, 
even when prolpetic to 1972.

Am I missing something here?

>
> For dates after 1972-01-01 I would say that NTP and POSIX are both
> confused
You bet

> about whether they are counting SI seconds or UT days.
Can you be more specific about what you mean by this? I think I 
understand your general meaning, that the "confusion", or the mis-match 
between the NTP and POSIX and UTC, lies with their methods of accounting 
for LOD. By "UT days" do you mean "86400 seconds per day"?

> Therein, of course, lies the basis of this LEAPSECS list.

Sure. Well, at least, therein lies *one reason* for 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.
Can you elaborate on this point as you've stated it? What exactly do you 
mean "switch from counting UT days to counting SI seconds"? This doesn't 
sound quite like what I understand the question(s) at WRC-15 to be.

>
>> 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.

I understand the proper SI second sprung into existence as of 1972. But 
it seems to me civil time as kept by NTP and POSIX before 1972 is 
measured in proleptically exrapolated SI seconds. Of course this scale 
does not correspond with "actual" historical date and time or UT1. What 
am I missing?

Thanks,

-Brooks
>
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