Seth Kurtzberg wrote:
Glynn Clements wrote: 
Bayley, Alistair wrote:

  
You also need to specify, or let the user specify, the order of
arithmetic.  I.e. adding a day and then a second is different from
doing it the other way around.
      

Only when leap seconds are involved. A month and a day would be a
better example.

  
Duration arithmetic is not commutative? Can you give some examples please,
because I'm not able to imagine any.
    

	April 30th + 1 month + 1 day = May 30th + 1 day = May 31st
	April 30th + 1 day + 1 month = May 1st + 1 month = June 1st
What reason is there to say May 30 + 1 day == May 31st?  May 30 + 1 day = June first.  So the answer is the same either way, as long as your result for May 30 + 1 day is correct.
oops, got my months mixed up, sorry about that.

The fallacy here is that month is a unit.  Your example proves that it is not.  Whether a day is a unit might be controversial (as noted in the thread, it depends on which assumptions you start from) but a month is definitely never a unit.

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