I have just spent three days searching for the right code, without any luck

I am not an Expert at programming and I know this is really easy.
What I basically want the spreadsheet to do is:
When market goes INPLAY, Start the timer.
The timer should be in seconds (only showing seconds in a cell)
I am going to use this, because then I know for how long the market has gone In play, and I can place my bets. This is a strategy for football, match odds.
The timer should stop counting automatically after a period of time (seconds).
Can anybody help me with this?