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From: eggert@grand.UUCP
Newsgroups: comp.os.vms
Subject: Re: Times and Time Zones
Message-ID: <23@grand.sm.unisys.com>
Date: Wed, 11-Nov-87 22:30:42 EST
Article-I.D.: grand.23
Posted: Wed Nov 11 22:30:42 1987
Date-Received: Sat, 14-Nov-87 10:50:56 EST
References: <871105071449.002@Mipl3.JPL.Nasa.Gov>
Reply-To: eggert@grand.sm.unisys.com (Paul Eggert)
Organization: Unisys Santa Monica
Lines: 43

In article <871105071449.002@Mipl3.JPL.Nasa.Gov> kka059@MIPL3.JPL.NASA.GOV
(Kurt Andersen) writes:

		...the system base date and time, which is 00:00 o'clock [sic]
		November 17, 1858 (the Smithsonian base data and time for the
		astronomical calendar)...

				VAX/VMS System Services Reference Manual
				page 9-2, Section 9.1 The System Time Format

	Unfortunately, no one at ... at the Smithsonian ... had *ever* heard of
	this ``astronomical calendar;'' not to mention that they were supposed
	to have been responsible for it!

I asked Bob Uzgalis, a friend who is writing a book about calendars but who
like me knows nothing of DEC time, about this.  He replied:

	I wouldn't wager, but I wouldn't be surprised if 0h November 17th 1858
	was an ephemeris date... and that it was an `astronomical standard' for
	ten years or so until precession and/or a new edition made another date
	appropriate.  After all the first edition of _Practical Astronomy with
	Your Calculator_ (Duffett-Smith) used Jan 0.0 1970 as its epoch
	date/time and the second edition uses Jan 0.0 1980.

	I vote for 0h BC 4713 Jan 1 (Julian Calendar, of course) as a good
	system base date.  Use one 32-bit word for the day (which will cover
	11.8 million years) and another for the time (resolution of 20
	microseconds).  This of course avoids problems with leap seconds, which
	fall into the gap between days and seconds.  Maybe DEC would be
	interested in starting the ball rolling?

He did not pick BC 4713 Jan 1 out of a hat.  It has been used as a common,
absolute time origin by astronomers for 400 years or so.  DEC seemingly did not
realize this and thought that some old Smithsonian ephemeris was permanent.

Obviously the new format should be UTC (ie GMT) as well.

While we're on the subject, a report in the latest _Science_ claims that some
pulsars are more accurate than the most accurate atomic clocks.  If this is
true, we could switch back to a natural phenomenon for our time standard!  A
corollary of the accuracy of the measurement is an upper bound on the mass of
the universe (otherwise gravitational waves would have spoiled the accuracy).
I urge time hounds to read this fascinating report.  --paul