What about CEM?
Could just use 0 to cancel, instead of creating a new command.
Well as a PC, I can imagine this and understand it 100%, sometimes things change in fast and unexpected way while you're doing your countdown, especially for countdowns that's for more than 10 seconds. I'm actually positive fully with this suggestion, but I prefer showing a key to press, so it cancels your countdown. Something like " Press 'C' to cancel your countdown (Don't take "C" itself, take the idea.