How to call setState without any triggering events?

I want to update the value of a counter variable after some fixed time indefinitely. Here is my code:

class Counter extends React.Component {
  constructor(props) {
    this.state = {
      myCount: 1    
  // Update state after every 15 seconds.
     this.setState({myCount: this.state.myCount + 1});
  }, 15000);

However, I get error about unexpected token with this component. How can I set state within the class properly without using any event listeners?