Re: [art] summary of updates - draft-time-touch

[Joe Touch <[e-mail]>]

Hi, Michael,.

Thanks for the detailed feedback.

Can you clarify as per below?

On 4/18/2017 3:01 PM, Michael Thornburgh wrote:
> Joe, all.
>
> i have several problems with this draft.  as others have mentioned, i think it's too long and complicated.  also, i feel that several of the definitions in section 3 Terminology are inaccurate or imprecise.
>
> below are my proposals for some of the Terms:
>
>  o Instant: a point in time.
Why is point more precise or accurate than moment?

>  o Time scale: the assignment of names to points in time.

A time scale is the set of all such assignments according to a single
system, not just the act of making the assignment, e.g.:

    Time scale: a system for assigning names to instants and intervals.

(the latter because time scales include names of intervals)

>  o Date: the name of an instant in a time scale.  alternatively: the name of a day in a calendar.
Those names are indicated as intervals from an epoch, so this can be
combined as:

    Date: the name of an instant in a time scale, indicated as an
interval from an epoch.

I disagree with the alternate definition. "Monday" isn't a date.

>  o Unit of time: this is unnecessary
OK.

>  o Epoch: a point in time that is the anchor/origin/beginning of a time scale.

Again, we ought to use "instant" here...  and "origin" seems the closest
IMO:

    Epoch: an instant used as the origin of a time scale

>  o Clock: an apparatus that measures (and typically reports) the passage of time.  a clock may report the time according to a time scale.
I disagree. Clocks don't report time passage; timers do. Clocks report
only the current value, and always in a time scale (what would a clock
be if it didn't? it might be a timer, but not a clock).

>  o Solar day: TL;DR.  please make more concise.  "mean solar day" would be useful to UT1 and therefore UTC.
I'm glad to hear suggestions. The trouble with this definition - and the
entire list, and the entire doc - is that for everyone who thinks its
too long, there's someone else who points out something to add.

UT1 isn't specified in terms of any mean.

>  o Tropical year: TL;DR.  is this necessary if you have "mean solar day"?

Both are needed because both are used for different definitions of a second.

>  o Second: the basic unit of time, having multiple different definitions:
>
>    * 1/86400 of a day / mean solar day.
>
>    * SI second: 9192631770 periods of the radiation corresponding to the hyperfine transition of the ground state of cesium 133 at 0K.  (note that sea level is NOT part of the definition of the SI second)
Good point.

It is TAI refers to sea level (you fix that later).

>  o Leap second: an extra second irregularly inserted into or removed from the Coordinated Universal Time time scale to keep it within 0.9 seconds of UT1.
I was hesitant to claim that this term was exclusive to UTC, but if we
agree, then OK.

>  o Leap day: this is unnecessary.
Probably...
> section 4.2 Time scales: 
>
>  o TAI (International Atomic Time): an ensemble coordinate time scale based on the SI second at mean sea level ("on the geoid"), determined after-the-fact by the weighted contributions of numerous atomic clocks in laboratories around the world, adjusted to account for gravitational time dilation caused by the altitude of each clock (among other gravitational effects).  TAI may appear to tick faster or slower than the SI second depending on the location (especially altitude) of the observer; therefore, free-running atomic clocks may diverge from TAI.  only approximations of TAI are available in real-time.
>
> i think it's important to make the above points about TAI because it illustrates why having an atomic clock on your desk or in your computer isn't good enough.
OK....

>  o UTC (Coordinated Universal Time): an approximation of UT1 based on TAI *adjusted* with leap seconds.
>
> later, in section 5, there is the implication that UTC is discontinuous.  UTC is continuous and ticks at the same rate as TAI.  leap seconds are not discontinuities.  the counting is non-uniform in that occasionally and irregularly there are minutes that don't have the *usual* number of seconds in them.
Good point. I'll fix that.

> POSIX time has discontinuities because it is fundamentally flawed.
I tried to clarify that POSIX is really two different things:
    - an API to a variety of time scales or approximations thereof (some
flawed)
    - Unix time, the counting of (undefined seconds) in fixed numbers of
seconds per day since a particular epoch

> there are other problems with the document, but i think dealing with them will be easier once the issues above are addressed.

OK - let me know what you think of the above and I'll work on a rev once
we converge.

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