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From: wyatt@cfa.harvard.EDU (Bill Wyatt)
Newsgroups: comp.misc
Subject: Julian Date (was Re: Sorting by date)
Message-ID: <763@cfa.cfa.harvard.EDU>
Date: Fri, 6-Nov-87 14:37:46 EST
Article-I.D.: cfa.763
Posted: Fri Nov  6 14:37:46 1987
Date-Received: Sun, 8-Nov-87 16:06:35 EST
References: <513@mtxinu.UUCP>
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 25

in article <513@mtxinu.UUCP>, tim@mtxinu.UUCP (Tim Wood) says:
> Summary: julian date, and others
> 
> The date format to which you referred is called "Julian date" (i.e.
> November 12, 1987 at 1:30PM == 198711121330).  I don't know why it's
> called "Julian".   This is a handy method of representing dates for
> indexing, but is inconvenient for date arithmetic. [...]

I've never heard of this format called Julian Date before, although it
may be distinct from the Julian Day Nmber (JDN), which is also called
(perhaps inaccurately) Julian Date. The standard internaltional
meaning of Julian Day Number is the number of days since GMT noon on
Jan 1, 4713 B.C. Thus, the JDN for Nov. 6th, GMT noon is 2,447,107.0.
The zero point is set far back in time so that essentially all JDN's
are positive. 

Lots of times people subtract off the 2,400,000 (giving dates back to the
1850's), and call it modified JDN or MJDN. Fewer digits to carry that way.
-- 

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