The way I did this in Swift 2.3 was:
let currentDate = NSDate()
let currentCalendar = NSCalendar.currentCalendar()
var startDate : NSDate?
var e
Found this on a different thread, but it was finally the most simple solution for me using Swift 4:
let previousDate = ENTER DATE HERE
let now = Date()
let formatter = DateComponentsFormatter()
formatter.unitsStyle = .brief // May delete the word brief to let Xcode show you the other options
formatter.allowedUnits = [.month, .day, .hour]
formatter.maximumUnitCount = 1 // Show just one unit (i.e. 1d vs. 1d 6hrs)
let stringDate = formatter.string(from: previousDate, to: now)