mxDateTime history (Re: mktime, how to handle dates before 01-01-1970 ?)

MRAB python at mrabarnett.plus.com
Sun Oct 11 17:55:15 CEST 2009


Rhodri James wrote:
> On Fri, 09 Oct 2009 13:39:43 +0100, Tim Chase 
> <python.list at tim.thechases.com> wrote:
> 
>>> Month arithmetic is a bit of a mess, since it's not clear how
>>> to map e.g. Jan 31 + one month.
>>
>> "Jan 31 + one month" usually means "add one to the month value and 
>> then keep backing off the day if you get an exception making the 
>> date", so you'd get Feb 31, exception, Feb 30, exception, Feb 29, 
>> possibly an exception, and possibly/finally Feb 28th.  This makes 
>> pretty intuitive sense to most folks and is usually what's meant.
>>
>> I've found that issues and confusion stem more from the 
>> non-commutative reality that "Jan 31 + (1 month) + (-1 month) != Jan 
>> 31 + (-1 month) + (1 month)" or the non-associative "Jan 31 + (1 month 
>> + 1 month) != (Jan 31 + 1 month) + 1 month" :-/
> 
> I'd hazard a guess that what we're actually seeing is people mentally
> rebasing their indices, i.e. counting from the end of the month rather
> than the start, which makes "the last day of January" and "January 31"
> not the same thing really.  Unfortunately we're very fuzzy about when
> we do things like this, which makes it hard on a poor programmer.
> 
And when someone says "January 30", do they really mean the day before
the last day of the month? Where would it end? :-)



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