issue with datediff for months

same same on cf8, too.
if first month of the 2 has 31 days the diff is 1, but if it is a month
that has 30 days the diff is 0:

<cfset dd = createdate(2009, 5, 31)>
<cfset dd2 = dateadd('m', 1, dd)>
<cfoutput>
#dd# - #dd2#<br>
datediff('m', dd, dateadd('m',1,dd) ) : #datediff('m', dd,
dateadd('m',1,dd) )#<!--- returns 1 --->
</cfoutput>


<cfset dd = createdate(2009, 6, 30)>
<cfset dd2 = dateadd('m', 1, dd)>
<cfoutput>
#dd# - #dd2#<br>
datediff('m', dd, dateadd('m',1,dd) ) : #datediff('m', dd,
dateadd('m',1,dd) )#<!--- returns 0 --->
</cfoutput>


not sure if it os odd or not though...

from datediff() docs:

"The DateDiff function determines the number of *complete datepart
units* between the two dates; for example, if the
datepart parameter is "m" and the dates differ by 55 days, the function
returns 1."

so, maybe, CF (and Java, too, i presume?) sees it as no FULL month is
encompassed when the first date has 30 days and the second has 31 days?

Azadi Saryev
Sabai-dee.com
http://www.sabai-dee.com/

JSBCHS wrote:
> In Coldfusion MX7:
>
> #datediff('m', '6/29/2009', '7/29/2009')# (evaluates to 1, which is good)
> #datediff('m', '6/30/2009', '7/31/2009')# (evaluates to 1, which is good)
> #datediff('m', '6/30/2009', '7/30/2009')# (evaluates to 0, which is a little
> confusing)
>
> This results in the following behavior:
>
> #datediff('m', '6/29/2009', dateadd('m',1,'6/29/2009') )# (evaluates to 1,
> which is good)
> #datediff('m', '6/30/2009', dateadd('m',1,'6/30/2009') )# (evaluates to 0,
> which is not good)
>

This is a known bit of ColdFusion date weirdness. When using the
datediff() function the difference is rounded *down* from the fractional
difference potentially down to the second. So since there are not a
full 31 days between your dates of 6/30/2009 and 7/30/2009 that there
needs to be to be a full calendar month of July, datediff() returns 0.

That is odd. Seems to give an unexpected answer when the first date is the last day of the month.

I tried various combinations of this:
Date3 = createdate(2009,2,27);
Date4 = createdate(2009,3,27);
writeoutput("#datediff("m",date3,date4)#");

What do the cf8 users see?