Charlie,
Good analysis. I think you're right about the docs being wrong.
What seems to be happening is that the function is counting the number of
FULL calendar weeks between the two dates. Elsewhere the docs describe
DayOfWeek as returning 1 for Sunday and 7 for Saturday, so it appears that
for CF a calendar week starts on a Sunday and ends on a Saturday. So for
Graham's dates there are 8 full calendar weeks, the first one starting
Sunday January 20, and the last one ending Saturday March 16th.
But then it talks about not counting date1 if it falls on a Sunday. It turns
out that that's not true: if you change Graham's start date to January 13th
( a Sunday) the number of weeks goes up to 9. On the other hand, it doesn't
seem to matter what day of the week date2 falls on, it only counts to the
last Saturday (full week) before that date.
Bob
----- Original Message -----
From: "charlie arehart" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Tuesday, January 15, 2002 3:20 PM
Subject: RE: [CFTALKTor] DateDiff question
> Well, while you'd think it should be 10, I'm more curious why it's showing
> 8, when according to the docs (which are pretty explicit about DateDiff
and
> use of the "ww" datepart for weeks) it should be 9:
>
> If interval is Week ("ww"), however, the DateDiff function returns the
> number of calendar weeks between the two dates. It counts the number of
> Sundays between date1 and date2. DateDiff counts date2 if it falls on a
> Sunday; but it doesn't count date1, even if it does fall on a Sunday.
>
> Some code demonstrates things
>
> <cfset date1="jan 14 2002">
> <cfset date2="Mar 18 2002">
> <cfoutput>
> #date1# falls on a #dayofweekasstring(dayofweek(date1))#<br>
> #date2# falls on a #dayofweekasstring(dayofweek(date2))#<br>
> Weeks between them: #datediff("ww", date1, date2)#
> </cfoutput>
>
> The result is:
>
> jan 14 2002 falls on a Monday
> Mar 18 2002 falls on a Monday
> Weeks between them: 8
>
> It reports 8, as you say, but I find that counting the number of Sundays
> between those dates, I get 9. And you want 10. :-)
>
> The mistake between 8 and 9 seems to be a bug, or a mistake in the docs (I
> think the latter, because if you simply count Mondays from Monday Jan 14,
> when you get to Mon Mar 18 you count 9. Now, you say you want 10. Here's
one
> way to get 10:
>
> Weeks between them: #ceiling(evaluate(datediff("d", date1, date2)/7))#
>
> This gets 9 with your dates, by dividing the number of days between the
> dates by 7, then the "ceiling" takes the integer part of the result (no
> fraction/remainder wanted) and raises it to the next whole number if there
> was a remainder. We don't want to round.
>
> Now, you wanted 10. I'd argue that this a semantic debate about what
> "between" means. But, from the algorithm in CF, it won't matter if you up
> the end date by a day. It still counts Sundays between the dates. In my
> code, since your second date always means "including this date", you can
> simply change the algorithm to add a day, as in:
>
> Weeks between them: #ceiling(evaluate(datediff("d", date1,
> dateadd("d",1,date2))/7))#
>
> Note that I've just slipped in a dateadd to add a day to the second date.
> This also demonstrates the reason for the ceiling. Without it, the answer
> would be 9.14285714286. And again you don't want to round down. The
ceiling
> makes it 10.
>
> Hope that helps. CF functions are really quite powerful!
>
> /charlie
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On
> Behalf Of Graham Faulkner
> Sent: Tuesday, January 15, 2002 1:20 PM
> To: [EMAIL PROTECTED]
> Subject: [CFTALKTor] DateDiff question
>
>
> Hi there,
>
> I have a site where I need to specify the amount of weeks a course is
> scheduled. For example, there is a course that meets every Monday night
> from January 14 until March 18. However, when I use #DateDiff("WW",
> StartDate, EndDate)# it calculates it as 8 weeks, instead of the 10 it
needs
> to be because there are classes on the start and end dates. Am I using
the
> function incorrectly? Should I be using something else to calculate this?
>
> Thanks in advance for your help.
>
> Blessings,
>
> Graham
>
> ------------------------------------------------------------
> Graham Faulkner, BBA
> President
> Global Audience Communications
>
> Web design with YOUR world in mind!
>
> Web: http://www.GlobalAudienceCommunications.com
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>
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> Cell: 519-496-0887
>
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