This is one of the few areas that was giving me some trouble with the methodology. I think I started a thread about it here, a few months ago.
At first, following DA's methodology to the letter, I used to populate my Calendar with actions to be done on a specified date, in addition to actions that had to be done on a specific day and time. That was wrong: I effectively ended up creating a To Do list and thus deviating from some fundamental concepts of GTD.
Later I discovered by reading a posting by Katherine, if I recall right, that I should limit entering these actions in my Calendar to the bare minimum number of occasions. Now I rely a lot on my Context Lists in deciding what the NA should be, and I find that I'm more productive (and less stressed) than by using the method that I described above.