[LEAPSECS] Common Calendar Time (CCT) -Brooks Harris

[Zefram]

Brooks Harris wrote:

>I'll suggest a tentative name of this new timescale - "Common

>Calendar Time (CCT)".


Decent name.


>B) Extrapolate an SI (1hz) timeline into the indefinite past,

>essentially declaring TAI and UTC proleptic timescales


You're conflating two different kinds of time scale here.  We have a
theoretical ideal time scale that by definition ticks perfect SI seconds:
Terrestrial Time (TT).  (Actually we have several such time scales, for
different relativistic reference frames; TT corresponds to an observer
on the rotating Terran geoid.)  TT is defined not just for the present
but also, proleptically, as far back as we care to go.  Or at least as
far back as the Earth has a well-defined surface.

TAI (and hence, modulo the labelling of the seconds, UTC) does not
tick perfect SI seconds.  TAI is the product of atomic clocks: it is
our attempt to technologically realise the theoretical TT.  We know
it doesn't perfectly match TT, and we have estimates of the errors.
TAI is steered in frequency to match TT, but not steered in phase, so
errors accumulate.  As it happens, since the TT/TAI synchronisation the
errors have been consistently in one direction.

TAI was essentially defined in mid 1958, when the atomic second was
defined and the 1958-01-01 synchronisation to UT2(USNO) was determined.
TAI can be extended backward proleptically, and in fact that 1958-01-01
epoch is a proleptic TAI time.  But, by definition, TAI can only be
extended as far back as there have been atomic clocks participating in
synchronisation activities: that is, to mid 1955.

So if you want a time scale that corresponds to TAI in the present
and extends back proleptically, you have to make an awkward choice.
Either you make it consistently match TAI and only extend back to 1955,
or to extend back further you have a two-part definition that switches
between a TT basis and a TAI basis sometime between 1955 and the present.
Or there's another option: you could formally use a TT basis for all time,
with the downside that in the present it diverges slightly from TAI.


>C) By declaring the anchor-point to existing TAI and UTC definitions

>as 1972-01-01T00:00:00Z we have imposed an *uncompensated* Gregorian

>calendar counting scheme on the proleptic part of the new timescale,

>making 0000-01-01T00:00:00Z the origin of the new timescale.


The proleptic Gregorian calendar is not limited in year range.
Negative CE year numbers are valid, commonly used in astronomy, and ISO
8601 provides syntax for them.  0000-01-01 was preceded by -0001-12-31.
There's no real origin here; the closest thing is the synchronisation
point nominated by ISO 8601 at 1875-05-20 (the signing of the Convention
of the Metre).


>E) Because "Leap Seconds" are at the center of the "kill Leap

>Seconds" debate,

...

>            we also rename (our beloved) "Leap Seconds".


Respelling isn't going to fool most of the people in this debate.


>- Leap Seconds don't (theoretically) only "leap" - they could also "drop"


The word "leap" doesn't carry any connotation about direction.


>- "correction" is more widely understandable than "compensated" or

>some more technical term.


"Correction" suggests a term that must always be added into certain
computations.  This is quite different from a leap, which is an one-time
irregularity in an otherwise regular sequence.  To be fair, what your
table labels "Earth Correction" does look like a correcting term.

Your table is incorrect in showing `UTC' times prior to 1961 and a `TAI'
time prior to 1955.  As discussed above, TAI is not defined for dates
prior to the use of atomic clocks.  UTC is likewise not defined prior
to the international agreement on how to steer time signals relative
to TAI.  It's legitimate to define some time scale that is continuous
with TAI and is defined for 1900, and you can then describe a 1900 time
on that time scale, but if you refer to actual TAI then you are subject
to its limitations.


>From 1961 to 1971 the correspondences you show between UTC and

TAI are not correct.  You have written the table as if UTC were
defined as exactly TAI - 10 s prior to 1972, which is not the
case.  The difference was 10 s only for the first half of 1972.
See <http://toshi.nofs.navy.mil/ser7/tai-utc.dat> for the expressions
defining rubber-seconds UTC.

You show your "Earth Correction" being a constant 10 s prior to
1972, and following the TAI-UTC difference thereafter.  This makes a
poor correction.  If the intent is to define a proleptic version of
modern UTC, you need to decide on dates for proleptic leap seconds.
For continuity your Earth Correction needs to at least approximately track
the TAI(TT)-UT1 difference.  (By "TAI(TT)" I mean TT - 32.184 s; that is,
TT adjusted to the TAI epoch, making a fake TAI that can be extended back
proleptically prior to 1955.  I presume this to be a good approximation
for anything that would match your "essentially proleptic TAI" intent.)


>G) We declare a new time axis that corresponds to the conventional

>"Offset from UTC" portion of "timezones"


"time axis" is a poor term for this.  De facto timezones already work
this way: they define a round-number offset from UT, and (regardless of
the legal basis time) everyone applies this offset to whatever form of
UT they have to hand, usually UTC if sub-second accuracy is available.


>I'll suggestion a name for this scale - "Local Time Offset (LTO)".


Decent name, and it can be fairly applied to the offsets that we already
manage, independent of the CCT concept.


>Local Time Offset declares 104 offsets from CCT on 15 minute

>increments from CCT-12:00 to CCT+14:00.


You will not in practice succeed in dictating a limit to either the
range or the resolution of local time offsets.  Each locality decides
its time for itself; there is no authority that can dictate otherwise.


>CCT-12:00, CCT-11:45, CCT-11:30, CCT-11:15, CCT-11:00, CCT-10:45,....

>CCT-00:30, CCT-00:15, CCTZ00:00, CCT+00:15, CCT+00:30, ....

>CCT+12:45, CCT+13:00, CCT+13:15, CCT+13:30, CCT+13:45, CCT+14:00


That's 105 offsets, not 104.  Fencepost.


>H) We define a "Nominal Day" to accommodate the nine possible lengths

>of the day, depending on instantiation of Earth Correction (Leap

>Seconds) and Daylight Savings -


Your table here implies two things.  Firstly, you are suggesting using
"24:" to represent the extra hour of a DST-extended day, and more
generally that DST be handled by counting an unusual number of hours
in the day and having an irregularity only at midnight.  This is in
some respects neater than the way DST is currently handled, in which
an interior hour of the day gets repeated or omitted; you're making it
more like the way leap seconds are (officially) counted.  But this runs
into the problem that it's not how DST is actually managed, and no one
has the authority to tell countries to manage DST (and offset changes
more generally) in this manner.  It's also incompatible with the EU's
arrangement of all countries, regardless of their particular offset,
changing offset simultaneously, so that the offsets between countries
remain constant.

Secondly, this table implies that the leap second, if there is one,
is applied at the end of the *local* day, rather than simultaneously
worldwide.  This would be a major change from how UTC operates, and would
lead to the unappealing situation of countries having time differing by
their LTO differences *plus a second* for a few hours.  It seems a bad
idea to mix the concerns of the worldwide base time scale and local time,
as this does.

-zefram