Clocks are introduced in level 8. When you light a fuse with a clock, it will freeze the timer for 10 seconds.
These are especially useful if you are running out of time and you still have a lot of rockets left to launch. If you have two or more clocks on the fuse you are about to launch, the timer will freeze for 10 seconds times how many clocks you have on the fuse.
When there are 5 seconds of frozen time left, "Unfreezing" will flash on the frozen lanterns.