That reminds me of something (With apologies to non-UK readers to whom it probably does not mean much.)

But that's the whole point! A countdown has to be stalled, repealed, reset, etc to make any adjustments. One can even argue that if you can do that to a countdown then it was not a very good countdown to start with, like the link above.

A count-up, on the other hand, does not need tweaking. It is open-ended by definition.