[tz] [PROPOSED] Be more consistent about UT vs UTC

Steve Allen sla at ucolick.org
Tue Nov 7 17:31:05 UTC 2017

On Wed 2017-11-08T01:08:34+0800 Philip Paeps hath writ:
> I wonder if it wouldn't be easier to simply note "where we write UTC, we
> actually mean UT when referring to timestamps before UTC was introduced".

This makes it tricky to include contemporary documents which originally
used one or the other.  In many of those cases the usage was uninformed
and often the term used was inappropriate for the actual meaning.

The inception of all the forms of UT (UT0, UT1, UT2), and their usage,
is available in issues of BIH's Bulletin Horaire from 1955.  See
So definitions for terms related to UT are easy to pinpoint.

It is much harder pinpoint UTC.  The inception of UTC in print is
found in Bulletin Horaire, Series J, No.  1 (1964 Jan/Feb) which was
not actually prepared for printing until after 1965-05-01.  That first
use in print does not declaratively define UTC, but rather employs a
term which had evidently been in informal use since some point after
1961 January.  Nevertheless, most publications and radio broadcasts
continued to use the term GMT or UT (and often local legal civil time)
into the 1970s even when they were broadcasting signals that would
better have been called UTC.  The first documented use of the term UTC
in radio broadcast time signals is by WWV and WWVH starting
1974-01-01.  There is no formal definition of the term UTC until
the 13th Plenary Assembly of the CCIR approved Recommendation 460-1
during 1974 July.

Nevertheless, the distinction between UT,UT0,UT1,UT2 and UTC is clear
from the proceedings during the decades from the 1950s through the
1970s.  UT is a time scale derived directly from observation of the
rotation of the earth in a way that subdivides days of the calendar.
UTC was always a time scale derived from atomic chronometers and
adjusted to remain within specified close agreement to UT.

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