title: Adding two number variables returns NaN
body: <p>Working on a simple game, using delta time for the first time.</p>
  
  <p>Delta time (dt) is created in this function:</p>
  
  <pre><code>function main() {
      var now = Date.now();
      var dt = (now - lastTime) / 1000.00;
  
      update(dt);
      render();
  
      lastTime = now;
      requestAnimationFrame(main);
  };
  </code></pre>
  
  <p>gameTime (my second variable) is just created as a var.</p>
  
  <pre><code>var gameTime = 0;
  </code></pre>
  
  <p>The problem comes when I try to add dt to gameTime, here:</p>
  
  <pre><code>function update(dt){
      gameTime += dt;
  };
  </code></pre>
  
  <p>this returns NaN, whatever I seem to do to it. I can display gameTime, and add to it using ++, and I can display dt (0.017 usually), but as soon as I add one to the other, I get NaN.</p>
  
  <p>How can I fix this?</p>