Best method for countdown timer ?

Hi everyone ! How are you ?

So, I made a countdown timer in my game using RPC calls every frame, and it works perfectly. But I don't think that is the best method to do. Indeed, my script needs so to call many RPC at everyframe.
I tried a system which call one RPC to start a local timing on each client, but this has synchonization problems and/or doesn't start on all clients.

How can I make a better countdown timer ??
I heard about OnPhotonSerializeView, but I don't know how to use it in my case and if it would be usefull.

Here's my script