Sync timer and Rejoin

I want to have a countdown timer for every player in the room, when time=0 time up, and game over. for this requirement, I need to sync the timer between each client to prevent timer still have left when another client is 0 already. And also, if timer is still not become 0 and some player disconnect, who can rejoin later, if timer turn 0 already, player cannot rejoin and lose or left. How can I figure out how these code work?Thank you