Thanks for the tip! Now I have:
tell application id "DNtp"
set sel to (selection as list)
my performSmartRule(sel)
end tell
on performSmartRule(theRecords)
tell application id "DNtp"
repeat with theRecord in theRecords
set theName to the name of theRecord
set creation date of theRecord to my getDate(theName)
end repeat
end tell
end performSmartRule
on getDate(theName)
set sedString to quoted form of "s/^.* \\([A-Z][a-z]\\{2,3\\} [0-9]\\{1,2\\}, [0-9]\\{4\\}\\)/\\1/"
set theString to do shell script "echo " & quoted form of theName & "| sed " & sedString
set theDate to date theString
return theDate
end getDate
And this works perfectly when I select records and run from the script editor. However when I try to run this as a smart rule, I get invalid argument date.