Hi this is how you make a timer when the game starts.
First you need these things: Life cycle, repeater, and a counter
First take the lifecycle and set it to game start
The attach a wire from the lifcycle to the repeater. (press z)
configure it like so
then go to the counter.
go to target on the counters menu and set the counter to what ever time you want it to reach. make sure you click yes.
now connect a wire from the repeater to the counter. configure it how you like to make the timer go up or down.
make sure your repeater activates every second which can be found here.
and thats how you make a working timer. If you want it to end the game when its the timer is over then get a end game device and connect the counter to it.