From the Finder dictionary for Class item: ‘creation date date [r/o]’ = read/only
I think your out of luck using the Finder to do this.
You can use the ‘apply catalog info: Set the finder info for a file’ part of Akua Sweets oaxen, as in this script (which works, open event log to see result quickly):
set theFile to choose file
tell application "Finder"
activate
set item_info to info for theFile
set creation date of item_info to date "Saturday, January 1, 2000 12:00:00 AM"
apply catalog info item_info to theFile
end tell
info for theFile