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From: hughes@math.berkeley.edu (Eric Hughes)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: Looking for D.O.W. algorithm
Keywords: weekday
Message-ID: <1989Sep18.165034.19753@agate.berkeley.edu>
Date: 18 Sep 89 16:50:34 GMT
References: <6470@hubcap.clemson.edu> <1839@sactoh0.UUCP> <766@chyde.uwasa.fi>
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Reply-To: hughes@math.berkeley.edu (Eric Hughes)
Organization: UC Berkeley Math Dept
Lines: 19
In-reply-to: ts@chyde.uwasa.fi (Timo Salmi LASK)

In article <766@chyde.uwasa.fi>, ts@chyde (Timo Salmi LASK) writes:
>This is a subject that seems to pop up about every weekday:-)  The
>bios call only returns the *current* weekday, but for *any* weekday
>an algorithm is needed.  

The standard trick under MSDOS is to save the current date, then set
the date to the desired one, get the day of the week, and finally
reset the date to the real one.

Alternatively, use a julian date routine, divide the resulting number
by seven, and use the remainder to get the day of the week; this
requires calibration to a known date-day pair.

Oh, and if you want to do this for dates previous to the 15th century,
you'll need to take into account various calendar adjustments that
were made. :-)

Eric Hughes
hughes@math.berkeley.edu   ucbvax!math!hughes