Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!zaphod.mps.ohio-state.edu!swrinde!ucsd!ucbvax!van-bc!balden
From: balden@van-bc.wimsey.bc.ca (Bruce Balden)
Newsgroups: comp.databases
Subject: Re: Calendar Algorithm
Message-ID: <1424@van-bc.wimsey.bc.ca>
Date: 14 Aug 90 20:24:08 GMT
References: <3091.26c69b52@cc.curtin.edu.au>
Reply-To: balden@van-bc.UUCP (Bruce Balden)
Organization: USENET Public Access, Vancouver, B.C., Canada
Lines: 31

Keywords: 

>	SUBROUTINE JULIAN(D1,M1,Y1,D2,M2,Y2,F,J2,W2,T2)
>C	ALGORITHM DERIVED FROM HART: SOFTWARE - PRACTICE AND EXPERIENCE
>C	Julian conversion and inverse -- takes into account
>C	J2, the Julian date, is the exact number of days since
>C	D-M-Y, the Gregorian calendar, was adapted in the United

Most elegant calendar algorithms depend on the fact that the calendar is
much more regular if years are viewed as beginning in March, i.e. the 
zero'th day of the year is March 1 and the last day of the year is 
Feb 28 or 29.  This allows the simple formula floor(30.6*MONTH+0.5) to
calculate the day displacement.  This "simple" formula can be complicated
a little to avoid floating point arithmetic.  On this simple observation and
the idea of using Julian day numbers hang all elegant calendar algorithms.

The usefulness of this approach is no accident: The year orginally really
did begin on March 1.  Look at the names of the months of the year:

November "ninth month", September "seventh month", etc.





Bruce Balden

Grand Thaumaturge.


Have a nice one.