BTW I've never understood the difference between sorting files by date added and date created…
I don't remember quite correctly but that was one of the reasons I ditched Finder completely and found Path Finder.
Basically, one is the date the file was created on the remote server (i.e where you downloaded the file from), and seems to be the one Finder enjoys sorting by.
So I had a lot of downloaded files and no way to tell when I exactly downloaded them as they showed dates years ago (when they were created).
You can see solutions to this problem
here and
here, or even
here.
So, instead of playing around with the piece of script I pasted you could just "fix" the problem from the root.
If you wanna play with Applescript like you did, then something like
this may help, but it's not for the faint of heart (but in your case, shall you do what's mentioned in the last paragraph, this right here would mean the solution for all files in your download folder, not just the ones you add from now on; now meaning whenever you implement what is instructed in the last paragraph).
PD:
In a broad sense, I'd explain what you ask like this:
I create a file and upload to a server (apple.com/iTunesInstaller.dmg). Creation date is 13-Dec-2012.
Then you download it today. Modification date is probably 03-Jun-2013, since the download process involved adding some attributes (read, write for the current user), hence the file was modified.
Added date is I think, where it got added to the filesystem so it should be the same as Modification date.
When you modify the file tomorrow, then Creation and Added date will stay the same, but Modification date will change.
There is also Last accessed date, which is where you last opened the file (?).
Anyway, its confusing and I don't give it too much thought, all I know is that I sort my files by Modification date always.
Please notice that some servers may or may not return the original attributes of the file (it also depends on how you copied the file to the remote server) so Creation date not always means the real creation date of a given file.