MMO Timer?

I want to make a MMO game like grepolis and i want to make raw materials to produce.
For example the wood is 0 and after 60 seconds became 10, but when you close the google the game produce the materials.

I might use a Finite State Machine, and switch states every time the timer hits zero. You can make a simple timer by saying

 private float _timer = 60.0f;

 public void StartTimer() {
     _timer -= Time.deltaTime

     if(_timer =< 0)
           _timer = 0;

     if(_timer = 0)
           //Do Something
     }