NSDate extensions for time measured from system restart

NSDate+SystemRestart.swift

/// seconds from the reference date to the system's last restart
private let secondsFromReferenceDateToRestart:NSTimeInterval = {
  /* we pretend these next two statements take place atomically, although in fact they do not, and the fact that clock progresses between the two statements will introduce miniscule error into our calculation */
  let secondsFromSystemRestartToInit = NSProcessInfo().systemUptime
  let secondsFromReferenceDateToInit = NSDate.timeIntervalSinceReferenceDate()
  let retVal = secondsFromReferenceDateToInit - secondsFromSystemRestartToInit
  return retVal
}()

extension NSDate {
  /**
   Returns an `NSDate` intialized relative to the moment of the sytem's last restart, as reported by `NSProcessInfo.systemUptime`.
   
   - parameter timeIntervalSinceSystemRestart: seconds from the system's restart until the moment we want the new object to represent.
   
   */
  convenience init(timeIntervalSinceSystemRestart ti:NSTimeInterval) {
    let secondsFromRestartToNow = ti
    let timeFromReferenceDateToNow = secondsFromReferenceDateToRestart + secondsFromRestartToNow
    self.init(timeIntervalSinceReferenceDate:timeFromReferenceDateToNow)
  }
  
  /**
   seconds from the system's last restart to the moment represented by the current receiver
   */
  public var timeIntervalSinceRestart: NSTimeInterval {
    let secondsFromReferenceDateToThisMoment = self.timeIntervalSinceReferenceDate
    let secondsFromRestartToThisMoment = secondsFromReferenceDateToThisMoment - secondsFromReferenceDateToRestart
    return secondsFromRestartToThisMoment
  }
}