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From: richard@locus.com (Richard M. Mathews)
Newsgroups: news.software.b
Subject: Re: CNews - now a pedantic software! : -(
Message-ID: <richard.671785266@fafnir.la.locus.com>
Date: 16 Apr 91 07:01:06 GMT
References: <3313@crdos1.crd.ge.COM> <5077@gazette.bcm.tmc.edu> <1991Apr09.160444.25262@looking.on.ca> <1991Apr9.185748.11721@mp.cs.niu.edu> <1991Apr9.221012.19572@visix.com> <9104122026.57@rmkhome.UUCP> <1991Apr13.233035.8779@looking.on.ca>
Organization: Locus Computing Corporation, Los Angeles, California
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brad@looking.on.ca (Brad Templeton) writes:

>However, perhaps a floating point number of days is the most logical date
>interchange format.   This is clean, although FP minutes has the advantage
>that people who want to just use integer math and don't care about seconds
>have an easy out.  Just about all news software would be fine with long integer
>minutes, and those who really want accurate time could use the fractional part
>if they desired.

I am not taking a position for or against such a proposal.  I would like
to point out, however, that such a standard already exists.  Astronomers
and others have long had a need for dates with which calculations are
easy.  They use fractional days to whatever precision is appropriate,
and they use a standard epoch.  If you want to pick an epoch, why not
one which has already gained acceptance somewhere?

The system used by astronomers is the Julian Date.  In
	Meeus, Jean, _Astronomical_Formulae_for_Calculators_,
	(Willman-Bell, 1985).
it says
	The Julian Day begins at Greenwich *noon*, that is at 12h
	Universal Time (or 12h Ephemeris Time, and in that case the
	expression Julian Ephemeris Day is generally used).  For
	example, 1977 April 26.4 = JD 2443259.9.

The epoch turns out to be JD 0 == January 1.5, 4713 B.C. (assuming I
did the calculation right, but that date sounds about right).  I have
no idea why this number was chosen (except for a vague memory that it
was someone's idea of the time of the creation of the universe).

Note that modern dates are up around 2 million.  A common convention is
to subtract off 2.4 million from these dates.  Note that JD 2400000.0
is November 16.5, 1858.  The convention of subtracting 2.4 million is
the origin of the Modified Julian Day mentioned in another posting:

From: nreadwin@micrognosis.co.uk (Neil Readwin)
>|> VMS has a really strange one [epoch].  I think it starts on Feb 14, 1758.
>It starts on November 17, 1858, which is the base of the Modified Julian Day
>system adopted by the Smithsonian Astrophysical Observatory (SAO) in 1957 for
>satellite tracking.

Meeus's book provides algorithms for conversion from Gregorian Date to
Julian Date and back.  I've modified what is there slightly to make this
more like code than the verbal recipe there.  In the following, INT(x) means
the integer part (round towards negative infinity: INT(-1.5) == -2).  This is
not intended to be in any particular programming language though it looks a
lot like C.  This is untested, so I could easily have translated something
wrong from the book.

    To convert Gregorian (with year -1 == 2 B.C.) to JD
	Let "y" be the year, "m" be the month (1 to 12), and "d" be the day
	(including any fraction).  Calculate the Julian Day, "jd".

	if (m <= 2) {
		y = y - 1;
		m = m + 12;
	}
	if ((y > 1582)
	 OR (y == 1582 AND m > 10)
	 OR (y == 1582 AND m == 10 AND d >= 15)) {
		a = INT(y/100);
		b = 2 - a + INT(a/4);
	}
	else
		b = 0;

	jd = INT(365.25 * y) + INT(30.6001 * (m + 1)) + d + 1720994.5 + b;

    To convert positive JD to Gregorian (with year -1 == 2 B.C.)
	Let "jd" be the Julian Day (including fraction).  Calculate
	"day", "month", and "year".

	z = INT(jd + .5);

	if (z < 2299161)
		a = z;
	else {
		a0 = INT((z - 1867216.25) / 36524.25);
		a = z + 1 + a0 - INT(a0/4);
	}
	b = a + 1524;
	c = INT((b - 122.1) / 365.25);
	d = INT(365.25 * c);
	e = INT((b - d) / 30.6001);

	day = b - d - INT(30.6001 * e) + jd + .5 - z;
	if (e <= 13)
		month = e - 1;
	else
		month = e - 13;
	if (month <= 2)
		year = c - 4715;
	else
		year = c - 4716;

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