From: Pit Capitain <pit@...>
Date: 2005-09-27T22:19:20+09:00
Subject: Re: Time interval

Gavin Kistner schrieb:
> On Sep 27, 2005, at 2:51 AM, Robert Klemme wrote:
>>
>> No, I mean because of the underlying model (our calendar).  Months  have
>> different lengths.  Same for years and quarters.
> 
> For example:
> 
> require 'date'
> d1 = DateTime.parse( "2/1/2000" )
> d2 = DateTime.parse( "3/2/2000" )
> puts d2-d1 #=> 30
> 
> Should the human-readable difference in that time say:
> 1 month, 1 day
> 30 days
> 1 month
> ?
> 
> How about:
> d1 = DateTime.parse( "2/1/2000" )
> d2 = DateTime.parse( "2/1/2001" )
> d3 = DateTime.parse( "2/1/2002" )
> puts d2-d1 #=> 366
> puts d3-d2 #=> 365
> 
> Should those both say "1 year", or "1 year, 1 day" for the first?
> 
> If you standardize and say "All 'months' are 30 days long, all  'years' 
> are 365 days" then you can run into situations like the above  where the 
> obvious difference to a human is different from the  computed 
> difference. This only falls down when you try to go beyond  weeks;  if 
> all your time intervals are short enough that formatting  them as "3 
> weeks, 4 days, 5 hours, 27 minutes" is acceptable enough,  then you're 
> good to go.
> 
> Alternatively, you need to keep year/month/date information along  with 
> the endpoints of your interval, and do piecewise differencing.

Recently I've been representing time intervals by two numbers: months 
and seconds. With this representation it is possible to differentiate 
between "1 year" and "365 days":

   1 year = 12 months (and 0 seconds)
   365 days = (0 months and) 365 * 24 * 60 * 60 seconds

Regards,
Pit