AJAX countdown timer that can be reset

Question

I'm looking to implement a count-down timer that you see on auction sites with "overtime". So, that's a timer that counts down to a specific date/time but if a bid comes in, then X seconds are added to the timer and the time continues clicking down. Let's say X=10 sec and 5 bids come in when the counter is at 00:03, then the new counter would ready 00:53, and so on. In theory, the counter could live on forever but will eventually hit Zero (no more bids).

Best way to program this? I'd love to see a complete C# solution but have also used jQuery. The timer needs to be aware of a mechanism to tell it when X seconds need to be added.