RE: doing date arithmetic in CL

["Shaw, David" <dshaw@xxxxxxxxxxx>]

-----Original Message-----
From: Joel Fritz [mailto:JFritz@sharperimage.com]

I agree with you about date calculations in CL and the new Y2K myth, but in
all fairness, I think a lot of engineering calculations are based on
algorithms that work most of the time or at least produce results that are
"close enough."  e.g. Newtonian mechanics. The leap year algorithm that uses
4, 100, and 400 is only a pretty darned good approximation, admittedly a lot
better than the one that only uses 4.

I had an extra day off and need to slough off some of the extra energy
through pedantry. <g>

---------------------

Joel,

Well, if we're going to be pedantic, let's pick a nit: the 4, 100, 400 rule is
exact for the Gregorian calendar for the range from AD 1 to AD 3999.  The
Gregorian calendar is where the approximation exists - it attempts to maintain
its anchor point in approximately the same part of the earth's orbit around the
sun from year to year.

Oh, that AD 3999 thing?  Some astronomers say that to maintain the approximation
years divisible by 4,000 should not be leap years, unless they are divisible by
20,000.  I've never been able to verify whether that is "official" in the
Gregorian calendar or not - I'm not even sure who would decide.

Dave Shaw
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