I use t0 and t1 as timestamps for the last two clicks. If the user clicked once within the first second of the game and t0 and t1 were 0 it would count as clicking three times. In order to avoid that I initialize these values with -16, i.e. one second before the game started. In order to make the code work you could have also taken any number less than -16. That wouldn't make a difference here.

Please note that I changed the code above. I moved assignments into the else clause. Otherwise the user could cheat when clicking 5 times consecutively.