Swift timeInterval formatter - macos

Question

I can get the date in de date picker, calculate the time left and put in a label. This gives seconds:
let timeLeft = datePicker.dateValue.timeIntervalSinceNow
countingLabel.stringValue = "seconds left: \(timeLeft)"
How to format "timeLeft"?
I tried this and it does not work:
var formatter = NSDateFormatter()
formatter.dateFormat = "EEEE, yyyy-MM-dd hh:mm:ss"
let timeLeft2 = formatter.stringFromDate(timeLeft)

Answer 1

You can use NSDateComponentsFormatter. From the
documentation:
An NSDateComponentsFormatter object takes quantities of time and
formats them as a user-readable string. Use a date components
formatter to create strings for your app’s interface.
Example:
let formatter = NSDateComponentsFormatter()
formatter.unitsStyle = .Full
formatter.allowedUnits = .CalendarUnitSecond | .CalendarUnitMinute | .CalendarUnitHour
let pickedDate = datePicker.dateValue
let now = NSDate()
let formattedTimeLeft = formatter.stringFromDate(now, toDate: pickedDate)!
// Example result : 1:06:50
There are some more output formats available, e.g.
.Abbreviated : 1h 6m 50s
.Short : 1 hr, 6 min, 50 secs
.Full : 1 hour, 6 minutes and 50 seconds
Update for Swift 4 and later:
let formatter = DateComponentsFormatter()
formatter.unitsStyle = .full // or .short or .abbreviated
formatter.allowedUnits = [.second, .minute, .hour]
let pickedDate = datePicker.date
let now = Date()
let formattedTimeLeft = formatter.string(from: now, to: pickedDate)!

Answer 2

To get timeLeft you should use two time components and get their differnece as follow:
let todayComponents = NSCalendar.currentCalendar().components(.DayCalendarUnit | .MonthCalendarUnit | .YearCalendarUnit | .CalendarUnitHour | .CalendarUnitMinute | .CalendarUnitSecond | , fromDate : NSDate())
var pickedDateComponents = NSCalendar.currentCalendar().components(.DayCalendarUnit | .MonthCalendarUnit | .YearCalendarUnit | .CalendarUnitHour | .CalendarUnitMinute | .CalendarUnitSecond | , fromDate : datePicker.dateValue)
pickedDateComponents.second = pickedDateComponents.second - todayComponents.second
pickedDateComponents.minute = pickedDateComponents.minute - todayComponents.minute
pickedDateComponents.hour = pickedDateComponents.hour - todayComponents.hour
pickedDateComponents.day =pickedDateComponents.day - todayComponents.day
pickedDateComponents.month = pickedDateComponents.month - todayComponents.month
pickedDateComponents.year = pickedDateComponents.year - todayComponents.year
let timeLeft = NSCalendar.currentCalendar().dateFromComponents(pickedDateComponents)!