Create a brush at the position where your start might be and convert it into a trigger_multiple. Place the target_timerstart somewhere in the room; not in the void. Link the trigger with the target and you should see a turquoise line (select the trigger first, then the target and press ctrl+k). If the player runs through the trigger the timer should start. If you want to create a checkpoint (yellow box) or a stop timer (red box) use the same method.
Just one more thing: If you want, you can make the start timer resettable (key: resettable; value: 1)
If the player runs through the trigger again after he has activated it, the clock restarts. But in my opinion a timer reset destroys the run.
For example: A lot of players like to grab the rocket launcher in the map, run back to the start, boost themselves through the timer again and save some seconds. That's normally not in the mind of the mapper. But you have to decide yourself if you want to use that function or not.
