CFML dates - not getting what I excpected

The following code yields;

tmdate1=*20130131.txt*

tmdate2=*20130131*
tmdate=*57014/06/04*
d:\inetpub\wwwroot\tat.net\development\control\phonedata\CallData_20130131.txt
57014/06/04

The asterisks are just for marking the output and looking for spaces.

<cfset tmdate1="#trim(right(attributes.filename,12))#">
<br>
<cfoutput>
tmdate1=*#tmdate1#*
<br><br>
<cfset tmdate2="#trim(left(tmdate1,8))#">
tmdate2=*#tmdate2#*
<br>
<cfset tmdate = "#dateformat(trim(tmdate2),'yyyy/mm/dd')#">
tmdate=*#tmdate#*
</cfoutput>
<br>
<cfoutput>#attributes.filename#</cfoutput><br>
<cfoutput>#dateformat(left(right(attributes.filename,12),8),"yyyy/mm/dd")#</cfoutput>

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lantervjAsked:
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SidFishesConnect With a Mentor Commented:
You can't pass 20130131 to date format as it interprets it as a numerical date value (I think)


try this (with a regex shortcut thrown in as a bonus :)

<cfset tmdate=rereplacenocase(attributes.filename, "[^\d]", "", "all")>
<cfset tmdate= "#left(tmdate, 4)#,#mid(tmdate,5,2)# , #right(tmdate,2)#">
#dateformat(tmdate, "yyyy/mm/dd")#

This works as long as the dates are always 8 chars long (ie: not 2013131)
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lantervjAuthor Commented:
Works great.  I'd like to know why, but can't stop and play now.
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lantervjAuthor Commented:
Great
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SidFishesCommented:
why is pretty simple

cf doesn't recognize it as a date

#isdate(tmdate)# returns NO

And often when cf doesn't understand format related things, it guesses....

and you get weird results.
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lantervjAuthor Commented:
So, your advice to me is;  Stop doing that?
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_agx_Commented:
> I'd like to know why, but can't stop and play now.

When you pass in something that looks like a number, ie "20130131" DateFormat treats it as the numeric representation of a date (similar to Excel).  That's different than interpreting the string as yyyymmdd.  With numeric dates:

 - 0 is the date 12/30/1899
 - 3 is the date 01/02/1900  (ie 12/30/1899 + 3 days)
- ...
- 20130131 is the date 06/04/57014 (ie 12/30/1899  + 20130131  days)

His code converts your string into something DateFormat recognizes as a date string.
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_agx_Commented:
Oops... didn't see the other replies before posting.

Like Sid said, CF does a lot of guessing - which is both good and bad. It makes for simpler code,  but if you don't understand *how* it guesses, you're in for a lot of headaches and unexpected results. Especially with dates.  

(Edit) Unfortunately part of the flexibility is that CF doesn't ask how it should parse date strings.  It just examines the input and takes its best guess.  The standard date functions can make sense of most U.S. date strings, but the safest bet is to stick with the non-ambiguous: yyyy-mm-dd format.
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lantervjAuthor Commented:
My life has been a living hell since 12/31/1999.   But, I get it.  I really do.  I just FORget it sometimes.  I rely on you guys to remind me.
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_agx_Commented:
No worries, happens to me too. We don't always notice the names of who's asking or remember their complete background.  So sometimes we may repeat stuff you already know. But I figure it's better to answer the question asked, than not, and let the asker ignore bits they already know.  Nothing worse than taking the time to ask a question and having someone ignore it.  (That's my personal pet peeve :)
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lantervjAuthor Commented:
Me too.  I didn't really mean anything by it.  I'm an old fart and thought I was through before y2k.  :)
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_agx_Commented:
Haha, if only. There's still the fun of daylight savings time changes and conflicting character encoding issues to play with. Just in case we weren't having enough fun ;-)
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