From: Steve Allen <[EMAIL PROTECTED]>
Subject: Re: [LEAPSECS] Introduction of long term scheduling
Date: Mon, 8 Jan 2007 22:57:23 -0800


> On Mon 2007-01-08T01:54:56 +0000, Zefram hath writ:
> > Possibly TT could also be used in some form, for interval calculations
> > in the pre-caesium age.
> Please do not consider the use of TT as a driver for the development
> of any sort of commonplace API.  In the far past no records were made
> using TT for the timestamp, and nobody ever will use TT except when
> comparing with ancient eclipse records.
> I agree that system time should increment in as uniform a fashion as
> possible, but amplifying reasons recently listed here I disagree that
> anyone should specify that the operating system uses TAI.  TAI is
> TAI, and nothing else is TAI.  Note that even in the history of TAI
> itself there have been serious discussions and changes in the scale
> unit of TAI to incorporate better notions of the underlying physics.
> GPS is not (TAI - 19), UTC is not (TAI - 33).  Millions of computers
> claiming to be running using TAI as their system time, even if they
> have rice-grain-sized cesium resonators as their motherboard clocks,
> will not make that statement true.  Instead it will simply obscure
> the concept of TAI much worse than it is misunderstood now.

All systems stating that they have a TAI or UTC time doesn't say they have
"the" TAI or "the" UTC. What is meant is that their local timescale attempts
to approximate TAI or UTC (respectively). This is indeed a fine point to
clarify. In that sense is GPS time not TAI - 19s, however the GPS timescale
is a TAI approximation offset by 19 seconds. The deviation of GPS timescale
from that of TAI is to be found in monthly documentation.

I don't think it is very helpfull for the larger audience to say that "your
computer does not have UTC", even if I agree technically with you. We just have
to be carefull with language. It is indeed usefull to use the terms "TAI time"
and "UTC time" and that their representations is implicitly always a local
representation and approximations of said timescales.

The one thing we must make clear that UTC is a derivate timescale from that of
TAI and that the difference between them does change over time.

So, while I agree that we should technically should be sure to keep things
very separatly, outside of a very small group of people, you benefit from
having a reduced set of terms in order to get the point though, and as it is
that is hard enought. The richness and fine detail of the terms we have should
not be lost (infact there is still room for improvements), but we should make
sure that we agree on how the reduced description is to be interprented.

I see nothing wrong with statements such as "my computer has UTC" since I never
will beleive that it will have THE UTC but rather some traceability to UTC, or
at least a very poor attempt (reset every now and then). I will naturally
challenge the traceability of that UTC representation (and indeed I have done
so to much amusement of myself and confusion of certain authorities and
govrement agencies).

Let's make sure we can win the wars we can win, and make a few smart comments
along the way to make people know that there is a more refined view if they
need to.


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