Path: utzoo!utgpu!watmath!iuvax!purdue!decwrl!wam.UMD.EDU!russotto
From: russotto@wam.UMD.EDU
Newsgroups: comp.sys.mac
Subject: Re: why 1904?  ms excel
Message-ID: <8909061947.AA00416@vs04csc.UMD.EDU>
Date: 6 Sep 89 19:47:14 GMT
References: <457a7553.1285f@maize.engin.umich.edu>
Organization: University of Maryland, College Park
Lines: 25

In article <457a7553.1285f@maize.engin.umich.edu> edt@caen.engin.umich.edu () writes:
>I have a question....
>
>From MS Excel User's Guide v 1.03 for the Mac:
>
>Excel stores dates and times as serial numbers from 0 to 49710, where
>0 is the number for January 1, 1904, and 49710 is the number for February 6,
>
>Why 1904?  I don't see why they chose that year rather than 1900 (which makes
>a little sense), or ibmpcs which have 1980 (the birth of the pc's) as 
>the start date.

The easy answer is that the Mac OS does it from 1904.  The deeper reason
for it, though, is that there is an easier leap year rule for dates
starting in 1904 and going to a date < 2100.  That is, for those
dates, a leap year is any year divisible by 4.  If you include 1900,
a leap year is any year divisible by four that is either not divisible
by 100, or is divisible by 400.

-- 
DISCLAIMER:  Not only does the University not share my opinions,
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	     Matthew T. Russotto
	     russotto@wam.umd.edu