I offered a draw and my opponent let time run out. The game was automatically drawn.

I didn't realize that if I offered a draw my opponent could just let time run out and the draw would be automatically accepted. I also didn't realize that I could not rescind a draw offer (though that shouldn't have surprised me).

You can rescind a draw offer when it is your move.

Ah! Of course, in this case my opponent never moved, so it was never my move again. :-)

yes correct

Actually, as I think on it, if the opponent moves then the draw offer is automatically rescinded. It was the other thing that really threw me -- that the opponent can just let the clock run out and the draw is accepted automatically. I wouldn't have offered it if I realized that s/he would do that.