From: xmlblog <xmlblog@...>
Date: 2005-09-28T02:21:43+09:00
Subject: Re: Time interval

While not in Ruby, I recently created a DateTime / Interval library in
Javascript based on the same ideas you've all been tossing around in
here. Ultimately, the language has very little impact on the design
questions that have been raised and there are some surprises in working
with dates as well.

When I wrote the Date Extras Javascript Library, I initially took the
approach of setting 30 days approx. = 1 month, 365 days approx = 1 year
(based on what I saw in Rails). But IMO the methods in Rails weren't
designed with the goal of high fidelity to the actual time. It seems to
me that they were designed to enable cool things like
time_ago_in_words.

In reality, chasing down accuracy in a general purpose Date library is
not an easy task. Subtle  design choices like what time of day is
Date.parse("12/1/2005") have big implications depending on the
application. In Date Extras I chose noon as the default time for some
methods because it made generating a calendar simpler.
Consider that doing something like this:
365.times do |i|
 puts Date.today.add_days(i)
end
will produce unintended results due to daylight savings time if
Date.today returns a date with 0 hours, minutes, seconds, and
milliseconds. Using a time like 12 Noon was easy because the clocks
tend to change at 1 or 2 in the morning.

I'll finish up by throwing an idea out there for the Interval
implementation and verturing an answer to the question of why date
subtraction yields a Rational. In JS, subtrating dates yields their
difference in milliseconds. This is because JS measures time as a
millisecond offset from 1/1/1970. To make my life easier, I created a
TimeSpan class whose initializer accepts milliseconds and provides
convenience properties to access that same interval in different
granularities. I also chose to use the absolute value of the
milliseconds in case the client code accidentally reversed the dates,
plus some might think it looks more natural to write:

elvis_lived = TimeSpan.new(Date.new("1/8/1935") -
Date.new("8/16/1977")).days

Anyway, if you're inclined to look at the library you can find it here:
http://www.xml-blog.com/articles/2005/09/19/date-extras-javascript-library-part-ii

Comments, suggestions, (flames?) welcome.