https://github.com/rails/rails/blob/master/activesupport/lib/active_support/duration.rb in Swift

Duration.swift

// Playground - noun: a place where people can play

import Foundation

extension NSTimeInterval {
    var seconds: NSTimeInterval {
        return self
    }
    
    var minutes: NSTimeInterval {
        return seconds * 60
    }
    
    var hours: NSTimeInterval {
        return minute * 60
    }
    
    var days: NSTimeInterval {
        return hour * 24
    }
    
    var weeks: NSTimeInterval {
        return days * 7
    }
    
    var months: NSTimeInterval {
        return days * 30
    }
    
    var years: NSTimeInterval {
        return days * 365.25
    }
    
    // MARK: Convenient Aliases, so 1.week works
    
    var second: NSTimeInterval {
        return seconds
    }

    var minute: NSTimeInterval {
        return minutes
    }

    var hour: NSTimeInterval {
        return hours
    }

    var day: NSTimeInterval {
        return days
    }

    var week: NSTimeInterval {
        return weeks
    }
    
    var year: NSTimeInterval {
        return years
    }
}

extension NSTimeInterval {
    var ago: NSDate {
        return NSDate(timeIntervalSinceNow: -self)
    }

    var fromNow: NSDate {
        return NSDate(timeIntervalSinceNow: self)
    }

    func since(_ date: NSDate = NSDate()) -> NSDate {
        return date.dateByAddingTimeInterval(self)
    }

    func until(_ date: NSDate = NSDate()) -> NSDate {
        return date.dateByAddingTimeInterval(-self)
    }
}

extension NSDate: Comparable {

}

public func <(lhs: NSDate, rhs: NSDate) -> Bool {
    return lhs.timeIntervalSinceReferenceDate < rhs.timeIntervalSinceReferenceDate
}

public func ==(lhs: NSDate, rhs: NSDate) -> Bool {
    return lhs.isEqualToDate(rhs)
}

public func +(lhs: NSDate, rhs: NSTimeInterval) -> NSDate {
    return lhs.dateByAddingTimeInterval(rhs)
}

public func +=(inout lhs: NSDate, rhs: NSTimeInterval) {
    lhs = lhs + rhs
}

public func -(lhs: NSDate, rhs: NSTimeInterval) -> NSDate {
    return lhs.dateByAddingTimeInterval(-rhs)
}

public func -=(inout lhs: NSDate, rhs: NSTimeInterval) {
    lhs = lhs - rhs
}

let d1 = 2.days.ago
let d2 = (1.day * 2).ago

d1.timeIntervalSinceReferenceDate
d2.timeIntervalSinceReferenceDate

d1 == d2
//
var date = 1.hour.ago

date += 20.minutes

let range = 2.5.hours.ago...date

let test = range ~= 2.hours.ago

let jan2_2014 = NSDate(timeIntervalSince1970: 1388620800)

3.hours.since(jan2_2014)