Passing variables between View Controllers using a segue

First, setup property/properties to hold your variables in your second view controller (destination).

class YourSecondViewController: UIViewController {
    var duration:Double?
}

Then have your button trigger your custom segue. Use your variable ('duration') as the argument for sender.

class YourFirstViewController: UIViewController {
    @IBAction func buttonTapped(sender: AnyObject) {
        self.performSegueWithIdentifier("MainToTimer", sender: duration)
    }
}

Finally, pass this sender data by overriding the prepareForSegue method:

override func prepareForSegue(segue: UIStoryboardSegue!, sender: AnyObject!) {
    if (segue.identifier == "MainToTimer") {
        let secondViewController = segue.destinationViewController as YourSecondViewController
        let duration = sender as Double
        secondViewController.duration = duration
    }
}

Yes, it is also possible to pass multiple variables and constants, again using the 'sender' parameter of prepareForSegue. If you have multiple data you want to pass in, put them in an array and make that array the sender.

SWIFT 3 From Swift 3, the method prepareForSegue(segue: UIStoryboardSegue!, sender: AnyObject!) has changed to prepare(for segue: UIStoryboardSegue, sender: Any?)

In the first ViewController place this (for modal segue):

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    let theDestination = (segue.destinationViewController as ViewController2)
    theDestination.Duration2 = Duration
}

Change ViewController2 to the name of the second ViewController. In ViewController2 create a class variable:

var Duration2 = (whatever the type - UInt8 I guess for time)

That's it. You will have in the value of Duration2 the value of Duration from the first ViewController.