From: mathew <meta@...>
Date: 2005-09-30T13:46:44+09:00
Subject: Re: Time interval

Randy Kramer wrote:
>    * (I guess this is the furthest off point): There seems to be an at least 
> "semi-official" designation of the weeks in a year--you will find some 
> calendars that number the weeks, and when I've checked, those numbers seem to 
> be consistent between calendars, even made in different countries.  (I think 
> the ones I've noticed, some years ago, were calendars from England, Austria, 
> and the US.)

You are probably looking for ISO 8601, which defines the algorithm for 
computing week numbers.

>    * Depending on the level of precision I wanted, I might calculate an 
> interval in weeks, then consider 4 1/3 weeks ~= 1 month, 13 weeks ~= 1 
> quarter, and 52 weeks = 1 year.  If I wanted to express some arbitrary number 
> of weeks in months, (e.g., 62, I'd divide by 4.34 and round off to the 
> nearest integer and call it that number of months.

If you want an interval to sufficient level of precision, the problem is 
insoluble.

Suppose you want 1 second precision. Well, unfortunately civil calendars 
have leap seconds, and the moments at which leap seconds will need to 
occur are not defined far in advance.  So DateTime(2020-01-01) minus 
DateTime(2000-01-01) can't be calculated to an exact number of seconds.

This is one reason why epoch dates are a very bad idea--it means that 
when a leap second is declared, you either need to change the meaning of 
existing data, or ignore it and let your time scale drift.

> If I wanted to express that in weeks, I guess my two choices would be to use 
> the semi-official designation of weeks (first bullet above) and get the 
> number of weeks by the difference in week numbers, and then depending on the 
> days of the week, approximate it to the nearest 1/2 (or actual number of 
> days).

I've a hunch that would lead to 'unexpected' results, because as per the 
standards some years have 53 weeks: the first week of 2004 started in 
2003, and the last week of 2004 ended in 2005.

> Or, calculate the actual difference in days, then divide by 7, call that 
> weeks, and the remainder in days (or rounded off to, say, 1/2 week.

Well, the question is really what someone means when they say something 
will take an interval of a certain number of weeks. If it's Thursday 1st 
and I say something will take 2 weeks, I mean it'll take until Thursday 
15th. I don't mean it'll take until Monday 19th (or Sunday 18th, 
depending on whether you think weeks begin on Sunday or Monday...)

I claim that most people who talk about durations in weeks mean "days 
divided by 7", without regard to whether the duration starts or ends on 
a week boundary.

(I've been writing calendar software for a sizeable chunk of my career. 
It's very hard to get right.)


mathew
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