title: Convert int to NSTimeInterval
body: <p>I'm making a game in Swift and I want the sprites to move faster as the time passes.</p>
  
  <p>I have a variable that increases in every second (It works with an <code>NSTimer</code>). I want to replace the duraton with this variable, in this line of code:</p>
  
  <pre><code>let action = SKAction.moveToY(120, duration: 3)
  </code></pre>
  
  <p>But I can't because the duration must be of type <code>NSTimeInterval</code>.</p>
  
  <p>How can I transform my variable (which is Int) to NSTime Interval so it can work properly?</p>