title: NSDate crash with successive actions
body: <p>I have the following code below that is meant to change a class var called "today" forward or backward by one day. It will work one time but then after that it crashes. It will do the same no matter if I press the left button or right button. What am I doing wrong?</p>
  
  <p>the var today is a class var initiated as .. <code>today = [NSDate date]</code></p>
  
  <p>Here is the method that crashes:</p>
  
  <pre><code>(IBAction)changeDateByOne:(id)sender{
  
      NSDate *newDay;
      NSDate *currentDay = today;
  
      NSTimeInterval secondsPerDay = 24 * 60 * 60;
  
      if(sender == leftButton){
           newDay = [currentDay addTimeInterval:-secondsPerDay];
  
      }else if(sender == rightButton) { 
           newDay = [currentDay addTimeInterval: secondsPerDay];
      }
  
      today = newDay;
  }
  </code></pre>