[LEAPSECS] Bulletin C and all that

Sun Jan 25 16:01:36 EST 2015

I agree with Michael.

The (proper) UTC timescale does not exist before 1972-01-01T00:00:00Z 
(UTC). That's the beginning of modern (solar) time. There was, or is, 
*by definition*, an initial 10 (integral!) second TAI-UTC offset at that 
moment. There is no agreed on a term for these initial 10 seconds - they 
are usually lumped into the term "Leap Seconds" - but that's misleading 
because the first real Leap Second occurred at (of course, actually 
immediately before) 1972-07-01T00:00:00Z (UTC).

Meantime, the epochs of NTP and POSIX are defined in terms of "UTC", but 
before 1972-01-01T00:00:00Z (UTC). They exist on a timescale I've been 
flamed for calling "proleptic UTC". This a timescale that extrapolates 
the Gregorian calendar counting method *uncompensated for Leap Seconds 
or anything else* into the (indefinite?) past before 
1972-01-01T00:00:00Z (UTC).

Note I've carefully qualified my character representation of date-time 
with the suffix "(UTC)". (By the way, why is there no accepted term for 
the idea of calendar date and time-of-day together, I wonder?) This 
because TAI is often also represented as a date-time but there is rarely 
a clear distinction made about what it means. Generally, one assumes the 
TAI timescale as represented as a date-time must be the Gregorian 
calendar counting method *uncompensated for Leap Seconds or anything 
else* because, of course, TAI certainly has no Leap Seconds.

ITU-R  TF.460-6 says -
B    International atomic time (TAI)
"... from the origin 1 January 1958 (adopted by the CGPM 1971)."

What, do you suppose, does "1 January 1958" actually mean? I guess it 
can't be on some "proleptic UTC" timescale because UTC doesn't yet 
exist, so it must be on an uncompensated Gregorian calendar timescale, I 
think. So I'll represent this as 1958-01-01T00:00:00 (TAI), with the 
suffix "(TAI)" to be sure there's no confusion with "(UTC)", and to 
indicate its a representation of the TAI timescale as an *uncompensated* 
Gregorian calendar timescale.

So, how many (integral) seconds between 1958-01-01T00:00:00 (TAI) and 
1972-01-01T00:00:00 (TAI)?

1972-1958 = 14 years * 365 = 5110 days + 3 leap year days = 5113 days * 
86400 seconds = 441763200 seconds.

So now the tricky question - how many (integral) seconds between 
1958-01-01T00:00:00 (TAI) and 1972-01-01T00:00:00Z (UTC)?

http://hpiers.obspm.fr/eoppc/bul/bulc/Leap_Second_History.dat says -

#    MJD        Date        TAI-UTC (s)
#           day month year
#    ---    --------------   ------
#
     41317.0    1  1 1972       10

What, exactly, does this mean? Do we interpret the MJD value as 
uncompensated Gregorian calendar, or should the conversion include the 
10 second initial offset somehow? What does "1  1 1972" mean, exactly? 
1972-01-01T00:00:00 (TAI) or 1972-01-01T00:00:00Z (UTC)?

So, again, how many (integral) seconds between 1958-01-01T00:00:00 (TAI) 
and 1972-01-01T00:00:00Z (UTC)? Is it 441763210 seconds or 441763190 
seconds? Put another way, is the "UTC epoch" 1971-12-30T23:59:50 (TAI) = 
1972-01-01T00:00:00Z (UTC)? Or 1972-01-01T00:00:10 (TAI) = 
1972-01-01T00:00:00Z (UTC). Or should it be represented as 
1972-01-01T00:00:00 (TAI) = 1972-01-01T00:00:00Z (UTC)?

I believe the correct answer is 441763210 seconds, or 
1972-01-01T00:00:10 (TAI) = 1972-01-01T00:00:00Z (UTC). I believe this 
because its consistent with other implementations. But, to tell you the 
truth, I'm not 100 percent sure - I really can't figure it out from the 
official specs Rec 460, BMPI and/or IERS because they never say what the 
heck they mean by their "date" representations.

I've seen many implementations that do not agree by 10 or 20 seconds, 
indicating I'm not the only confused reader. So I'm curious, what do 
other self-declared experts here on the list believe? What, exactly, is 
the origin or epoch of the UTC timescale?

And, Rob, what, exactly, does "1972  1" mean in your Leap Seconds table? 
1972-01-01T00:00:00 (TAI) or 1972-01-01T00:00:00Z (UTC)?

-Brooks

On 2015-01-25 12:00 PM, Michael Deckers via LEAPSECS wrote:
>
>   On 2015-01-25 14:58, Rob Seaman wrote:
>
>>  Please let me know about typos, suggestions, etc.  Needless to say this
> >  remains a prototype.
> ...
>> YYYY MM before  after  encoded crc IP              Decoded        flags
> > 
> ------------------------------------------------------------------------------
>> 1972  1      9     10 f8000a00  f5 248.0.10.245    -> OK 1972  1  10  
>> 1  (1, 0)
>
>   It would be incorrect to consider the discontinuity of the difference
>   TAI - UTC at the epoch when TAI was 1972-01-01T00:00:10 as a leap 
> second;
>   the difference increased by about 0.108 s, not by 1 s. Hence, a 
> timestamp
>   such as "1971-12-31T23:59:60.2Z" should not be made acceptable.
>
>   The first leap second occurred when UTC reached 1972-07-01; the 
> information
>   about a leap second says something about TAI - UTC both before and 
> after
>   the date referenced. At 1972-01-01, however, the information can 
> only say
>   something about TAI - UTC for TAI on or after 1972-01-01T00:00:10, but
>   nothing (correct) for smaller values of TAI.
>
>   Michael Deckers.
>
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