Press space or click to stop the black square. Try and stop the black square in the red square. See your data in the bottom of the screen. The green section means early. The red section means late. The yellow is frame perfect. Use this tool to improve your timing skill. How many frame perfect inputs you can get in a row?
Scratch runs at 30fps so this is a 1/30 second frame perfect and not a 1/60 second frame perfect.