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

[Tim Chase]

> 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" :-/

So yes, messy it is!

-tkc