Timesys note review

[Steve Allen]

On Tue 2018-12-04T09:52:31+0100 Markus Demleitner hath writ:
> I've changed the former UT1 in that table to:
>
>   \item[UT] Earth rotation time. We do not distinguish between UT0, UT1,
>   and UT2; applications requiring this level of precision need additional
>   metadata.  This should also be used to label GMT times in datasets covering
>   dates before 1972-01-01.

Dates for GMT are from 1925-01-01 through 1972-01-01 because before
then GMT in astronomy has a meaning which is offset by 12 hours.
The caution that earlier records in astronomical GMT must be converted
to UT should be included because that change is not universally known.

UTC is in step with TAI starting on 1972-01-01, before then requires
the lookup table of rate differences and sub-second steps.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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