Applescript to shutdown in 15 seconds

set userCancelled to false

try
    set answer to display dialog "Backup complete. Shutting down in 15 seconds." giving up after 15
on error number -128
    set userCancelled to true
end try

if userCancelled then

else if gave up of answer then
    tell application "Finder"
        shut down
    end tell
end if