title: Add a running countup display timer to an iOS app, like the Clock stopwatch?
body: <p>I'm working with an app that processes device motion events and updates interface in 5 second increments. I would like to add an indicator to the app that would display the total time the app has been running. It seems that a stopwatch-like counter, like the native iOS Clock app is a reasonable way to count time that the app has been running and display it to the user.</p>
  
  <p>What I'm not sure of is the technical implementation of such a stopwatch. Here's what I'm thinking:</p>
  
  <ul>
  <li><p>if I know how long between interface updates, I can add up seconds between events and keep a count of seconds as a local variable. Alternatively, a 0.5 second interval scheduled timer can provide the count.</p></li>
  <li><p>If I know the start date of the app, I can convert the local variable to date for each interface update using <code>[[NSDate dateWithTimeInterval:(NSTimeInterval) sinceDate:(NSDate *)]</code></p></li>
  <li><p>I can use a NSDateFormatter with a short time style to convert the updated date to a string using <code>stringFromDate</code> method</p></li>
  <li><p>The resulting string can be assigned to a label in the interface.</p></li>
  <li><p>The result is that the stopwatch is updated for each "tick" of the app.</p></li>
  </ul>
  
  <p>It appears to me that this implementation is a bit too heavy and is not quite as fluid as the stopwatch app. Is there a better, more interactive way to count up time that the app has been running? Maybe there's something already provided by iOS for this purpose? </p>