You need two variables, one for minutes (A), one for seconds (B). A starts with 29 and B starts with 59. Then make a loop in which you decrease the value of B by 1 every second. Use the background text action part in order to display the current values. When B=0, decrease A by 1 and set B back to 59.
This loops goes on until A=0 and then call whatever action you want to happen.