Click the green flag to start. To stop/start timer, click the cat, or press space bar. That is all.
This timer only works if you are signed in with my Scratch username. So, if you want to use this code, you must Remix (copy it to your account) and modify the code so that it contains your Scratch username. Cloud variables are necessary for this code to run, however the code could easily be modified to work without cloud variables.