Multiplayer : Sync a timer

Hi,

I'm working on my multiplayer game and I was wondering how to sync the round timer. So my game is made of rounds and each take 30 seconds.

Even if I start the layout at the same time, that doesn't mean every peer will start at the same time. I tried to start when everyone send "ready" to the host, but when applying some lag, it's not sync.

Should I put the timer on an instance variable called "GameManager" and sync this one ?

Any idea ? Tips ?

I would also like to know that, and how a peer can change an instance variable and sync it.