如何用我的秒表显示确切的时间？

I've recently been working on making a Stopwatch app that is similar to the one on the stock iOS devices. I have the timer working but I would like it to follow the same look and exact time like the stopwatch Apple has. As of right now I have it just showing the seconds. For what I know there has to be some kind of to convert the numbers to the 00:00.00 format.I've recently been working on making a Stopwatc