title: dateByAddingTimeInterval argument syntax
body: <p>If I send the method dateByAddingTimeInterval: to NSDate, like below:</p>
  
  <pre><code>NSDate *today = [NSDate date]; 
  NSDate *tomorrow = [now dateByAddingTimeInterval:24.0];
  NSDate *yesterday = [now dateByAddingTimeInterval:-24.0];
  
  NSArray *dates = [NSArray arrayWithObjects: today, tomorrow, yesterday, nil];
  
  NSLog(@"today's date is %@", [dates objectAtIndex:0]);
  NSLog(@"yesterday's date was %@", [dates objectAtIndex:2]);
  </code></pre>
  
  <p>I get this output: </p>
  
  <pre><code>...The first date is 2012-08-30 02:14:19 +0000
  ...The third date is 2012-08-30 02:13:55 +0000
  </code></pre>
  
  <p>Which is strange because the third date should have been 2012-08-29</p>
  
  <p>But... if I change the NSDate messages to:</p>
  
  <pre><code>NSDate *today = [NSDate date];
  NSDate *tomorrow = [now dateByAddingTimeInterval:24.0 * 60.0 * 60.0];
  NSDate *yesterday = [now dateByAddingTimeInterval:-24.0 * 60.0 * 60.0];
  </code></pre>
  
  <p>Why does adding the * 60.0...</p>
  
  <pre><code>...The first date is 2012-08-30 02:15:25 +0000
  ...The third date is 2012-08-29 02:15:25 +0000
  </code></pre>
  
  <p>make the output correct?</p>
  
  <p>Thank you.</p>