title: ReactJS not clearing interval properly
body: <p>I have a <code>setInterval</code> that I am trying to clear on <code>componentWillUnmount</code>.</p>
  
  <pre><code>componentDidMount() {
      setInterval(this.refreshTime, 1000);
  }
  
  componentWillUnmount() {
      clearInterval(this.refreshTime);
      console.log('cleared interval');
  }
  </code></pre>
  
  <p>And here is my <code>refreshTime</code> for good measure:</p>
  
  <pre><code>refreshTime = () =&gt; {
      this.setState({
          currentTime: this.renderCurrentTime()
      });
  }
  </code></pre>
  
  <p>The <code>console.log</code> logs, but I still get the <code>setState</code> error once the component unmounts.</p>
  
  <p><code>warning.js?8a56:36 Warning: setState(...): Can only update a mounted or mounting component. This usually means you called setState() on an unmounted component.</code></p>
  
  <p>Ideas?</p>