Hello, I want speedrun clockwork knight on emulator but I don't know how to start the timer (live split) and the game at the same time

You can usually set a negative time offset for a timer in most split software. You could set it to, say 5 seconds, and tab back into the emulator and start the game when it hits zero.

Thank you I will follow your advise

And How I stop the timer a the perfect moment when the timer need to stop ?

Other than setting up an auto splitter I think it's a best effort type of thing. Just trying to hit the split manually as consistent as possible.

Not sure if you are playing with a controller or keyboard but either way it should be possible to setup a key/button for the splits so that you don't need to tab out.