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[Bug 11616] UTC began at 1961-01-01T00:00Z, and there is no such thing as proleptic UTC. Therefore the Nero example is ill-defined.

From: <bugzilla@jessica.w3.org>
Date: Mon, 31 Jan 2011 16:53:07 +0000
To: public-html-bugzilla@w3.org
Message-Id: <E1Pjwzz-0005LW-R1@jessica.w3.org>
http://www.w3.org/Bugs/Public/show_bug.cgi?id=11616

Warner Losh <imp@bsdimp.com> changed:

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--- Comment #4 from Warner Losh <imp@bsdimp.com> 2011-01-31 16:53:07 UTC ---
Actually, when UTC started is a lot more complicated than a simple date.

UTC as we know it today started on 01-Jan-1972T00:00:00Z.  This is when whole
seconds were inserted into UTC to keep in closely in sync with UT1.  Prior to
that date, frequency offsets and small steps of tens or hundreds of
milliseconds were inserted to keep them in sync.  That practice does date to
1961.  However, prior to that, the time scale now known as TAI ticked from 1958
forward.  Prior to that, well, time of day was based on a number of different
things.  UT1 is just a special variation of UT that was convenient to keep
since approximately 1900 (although many different definitions of UT1 existed),
so speaking of 'UT1' back 2000 years likely is a bit unwise as well.

Generally, as implemented by most time libraries, proleptic UTC doesn't do
anything close to a project back into the past of when leap seconds happened or
any of that crazy nonsense.  Instead, it tends to be implemented as more of a
proleptic TAI, where time ticks back in time without regard for leap seconds at
all.  But the idea is that these seconds that tick aren't SI seconds of a fixed
duration, but rather mean solar seconds of whatever era the time stamps are
from.  This matches historical practice and gets away from implementations need
to have complicated tables of numbers for the period of time we have good data
on these small differences (basically since 1600 or so), plus, there's no
standard for projecting time back like this: everybody agrees to paper over
these fine-details and go with what people used back in the era that time
information comes from.

If you are going to fix the Nero example, I'd say that it is midnight universal
time, and then explain that in prior eras time keeping was based not on fixed
SI seconds like today, but rather were 1/86400 of a mean solar day which has
varied throughout history by a few milliseconds.  Times expressed from this era
are 'universal' times.  But as a concept, universal times didn't really exist
prior to the various treaties that established Greenwich as the basis for the
longitude for the globe.

Of course, there is also the problem that timezones didn't exist prior to the
railroads promulgating them in the mid 19th century (in the US and UK, other
times elsewhere).  Prior to that, all times were either local solar time, or
whatever the big clock tower near where you lived said it was.  Each
observatory had its own time from about the 1500's forward, just to complicate
things further.

So putting this all together, the time example you gave for nero's birth most
likely should be interpreted as midnight local time on the date of his birth
had be been born somewhere near London.

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Received on Monday, 31 January 2011 16:53:09 GMT

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