You can use the (live:) macro:
(set: $time to 10)
(live: 1s)[(set: $time to it - 1)
(if: $time is 0)[(stop:)]]
This will decrease the timer by 1 every 1 second. Once it hits zero, it will stop and the $time variable will remain on zero. Then you can continue the (if:) statement to say 'Game Over' or something.