Re: Months are weird, was '3 Months Ago?'

[AGlauser@xxxxxxxxxxxx]

Booth Martin wrote on 17/05/2007 09:39:50:

> What bothers me is that dates don't round trip well.   Does March 31st 
> less one month, plus one month, equal March 31st?

It bothers me too, but I'm not sure there would be a sensible way to 
implement month arithmetic so that March 31 could round trip.  Suppose 
that

a = 2007-03-31
b = a - 1 month + 1 month

If b = a, then you would just have another inconsistency.

c = a - 1 month
d = 2007-02-28

I think that we can agree that  c = d.  But then you have

c + 1 month                       =  d + 1 month
(a - 1 month) + 1 month = (2007-02-28) + 1 month
2007-03-31                       = 2007-02-28 ......................... 
uh-oh!

Dates arithmetic will never behave like integer arithmetic, because 
'months' and 'years' are not really consistently defined.

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