Comparing dates in loop, then adding interval?

I've been really stuck on this one for a while. The user will set a custom alarm interval and then pick a date on the picker, and the app is set to display the next alarm. If they set it in the past, I want the app to add the interval by looping until it is in the future.

I've been playing around with the date comparisons but I can't seem to get anything to work. How would you guys accomplish this? Thanks!

You can add this extension to check if their date is before a given date:

extension NSDate
{
    func isGreaterThanDate(dateToCompare : NSDate) -> Bool
    {
        //Declare Variables
        var isGreater = false

        //Compare Values
        if self.compare(dateToCompare) == NSComparisonResult.OrderedDescending
        {
           isGreater = true
        }

       //Return Result
        return isGreater
    }


    func isLessThanDate(dateToCompare : NSDate) -> Bool
    {
        //Declare Variables
        var isLess = false

       //Compare Values
       if self.compare(dateToCompare) == NSComparisonResult.OrderedAscending
        {
            isLess = true
        }

        //Return Result
        return isLess
    }
}

And to use it, just do something like

let today = NSDate()//this creates a date object with current date
let isBeforeToday = today.isGreaterThanDate(aDate)//aDate is whatever date they pick from the picker

And then to add your interval you can do

today.dateByAddingTimeInterval(customInterval)//customInterval is in seconds

I'm not sure I know exactly what you want to do, but this should at least get you started with comparing dates and adding intervals to them