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From: lcc.jbrown@ucla-locus.arpa (Jordan Brown)
Newsgroups: net.unix
Subject: Re: Adding Time and Date
Message-ID: <3344@brl-smoke.ARPA>
Date: Wed, 27-Aug-86 12:34:32 EDT
Article-I.D.: brl-smok.3344
Posted: Wed Aug 27 12:34:32 1986
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Use ACM algorithm #199 "Conversions Between Calendar Date and
Julain Day Number".  Once you've converted the dates into julian
days (number of days since the founding of Rome, for trivia buffs)
the arithmetic is trivial.  When done, you convert back.

The algorithms are fairly short; I can type them in if you don't
have the Collected Algorithms available.