In Objective-C, converting from NSNumber to NSTimeInterval is straightforward by Type-Cast from double. Swift, however, needs more strict way, I would say.

NSNumber+MyExtensions.swift

import Foundation

extension NSNumber
{
    var timeIntervalValue: NSTimeInterval {
        get {
            return (self.doubleValue as NSTimeInterval)
        }
        // TODO: write a setter
    }
}