I can just recommend you such architect:
- Some array with options, that contains title and time of each countdown
- Only one function ctdt, that is running from setInterval (not setTimeout)
- Array with ids of intervals
- On document.ready run some function, which will analyze array with options, gets actual countdowns, which are not displayed on page yet and outputs as many items as needed.
- When some countdown is finished, you hide it and run the same to doc.ready function, which shows one more countdown, if its available.
It's more then just a question and it needs some time to realize, that's why I can't give you complete workni code, sorry.
Good luck!