Difference Between Two datetimes

[Steven D'Aprano]

On Mon, 28 Dec 2009 10:54:46 -0800, W. eWatson wrote:

> Ben Finney wrote:
>> "W. eWatson" <wolftracks at invalid.com> writes:
>> 
>>> Lie Ryan wrote:
>>>> what's strange about it? the difference between 2009/01/02 13:01:15
>>>> and 2009/01/04 13:01:15 is indeed 2 days... Can you elaborate what do
>>>> you mean by 'strange'?
>> 
>>> Easily. In one case, it produces a one argument funcion, and the other
>>> 2, possibly even a year if that differs.
[...]
> Well, it just seems weird to me. <g>. I'm modestly familiar with
> objects, but this seems like doing the following.
> 
> Suppose we have a module called trigonometry, trig for short. It
> contains lots of trig functions, and sort of uses the same concepts as
> datetime. Bear with me on that. Here's my imagined interpretive session:
> 
>  >> import trig
>  >> c=trig.sin(90.0) # arg is in degrees
>  >> print c
> trig.cos(1.0)
>  >> type(c)
> <type 'trig'>
>  >> value = c.value
>  >> print value
> 1.0
> I'd call that weird. Maybe in this case it is ... <g>


No, that's an invalid analogy, because trig functions are defined to 
return unitless numbers and so there is no point in distinguishing 
between the 1.0 you get from the sine of 90 degrees and the 1.0 you get 
from halving 2.0. They are the same thing.

But time is different. Not only do times have a unit, but we also 
distinguish between two different concepts: times as points on a 
calendar, as well as differences between such times.

A calendar time of 1000 seconds is not the same as a time difference of 
1000 seconds: 1000 seconds is 'Thu Jan  1 10:16:40 1970' according to the 
POSIX standard, Windows may pick a different moment for zero. But a time 
difference doesn't correspond to any specific moment in time at all, and 
represents a distance between two points on the calendar -- a duration.

Hence Python provides timedelta objects for working with time 
differences, and datetime objects for working with calendar times.

Of course, one might have chosen to take a different approach, and use 
(say) raw ints only for working with dates no matter whether they 
represent an absolute time or a relative time. Then the programmer would 
be responsible for interpreting 1000 as either 10:16:40 Jan 1 1970 or a 
duration of 16 minutes 40 seconds, whichever is appropriate. That's a 
legitimate design choice too, but not the one Python uses.



-- 
Steven