TimeDifference.swift

//While I'm not sure the details of your implementation, I'd probably recommend storing the start time as an NSDate(), 
//then instead of incrementing/decrementing from an Integer/Float/Double, you can just calculate the time since the initial
//date, then remove that amount of time from your totalTime. That way when the app reopens, the time calculation is always 
//correct. The calculation for determining how long ago an NSDate() is so light that it'll be nearly instantaneous.

import UIKit

class Timer {
    private var startTime: NSDate? // Recorded when the user starts the timer.
    private var totalTime: Double = 0.0 // If you're counting down, this is the total time designated by the user (e.g. 20 mins)
    private var currentTime: Double = 0.0 // Displayed time visible to user.

    private func startTimer() {
        startTime = NSDate()
    }

    private func updateTimer() {
        let timeDifference = startTime!.timeIntervalSinceNow // See http://stackoverflow.com/questions/4084341/how-to-calculate-time-in-hours-between-two-dates-in-ios

        if timeDifference >= totalTime {
            currentTime = 0.0
        }
        else {
            currentTime = totalTime - timeDifference
        }
    }
}

//source: https://teamtreehouse.com/community/swift-weather-app-reload-data