You use 2 variables, one for minutes and one for seconds.
Every second you will add 1 to the second variable and when the second variable is 60 you will change it back to 0 and add 1 to the minute variable.
You will then put those variables in a text object using "Set Text" to variable_minutes & ":" & variable_seconds.
Everything I just mentioned will only work with a condition like "player is alive", so it stops working when the player has died.
Hopefully this helps you a bit in the right direction