# Calendar Types

Seth Kurtzberg seth at cql.com
Tue Feb 15 00:45:25 EST 2005

```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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