I do this with a TextExpander snippet. I type "ddd" to insert @done(date time). Snippet template: @done(%date:yyyy-MM-dd% %H:%M)
If you're not a TextExpander user, any snippet manager that allows you to dynamically insert a date should suffice. If you're a Keyboard Maestro, Alfred or LaunchBar user, you can probably do something similar. And that's without having to script anything.