Format NSTimeInterval as hh:mm
let wholeSeconds = UInt64(elapsed)
let minutes = wholeSeconds / 60
let seconds = wholeSeconds % 60
let formatted = String.localizedStringWithFormat("%02d:%02d", minutes, seconds)
Format NSDate:
let formatter = NSDateFormatter()
formatter.dateStyle = .NoStyle
formatter.timeStyle = .ShortStyle
let formattedTime = formatter.stringFromDate(originalStartTime!)