Surely all you need to do is very simply:
1) Store the current Millisecs/1000 into a StartTime variable when you start the countdown
2) Calculate TimeElapsed=(Millisecs/1000)-StartTime
3) Calculate countdown with SecondsToWait-TimeElapsed
1) Store the current Millisecs/1000 into a StartTime variable when you start the countdown
2) Calculate TimeElapsed=(Millisecs/1000)-StartTime
3) Calculate countdown with SecondsToWait-TimeElapsed