NSDateComponentsFormatter's stringFromDate(_, toDate:) returns nil

From the headerdoc:

/* Bitmask of units to include. Set to 0 to get the default behavior.
   Note that, especially if the maximum number of units is low, unit
   collapsing is on, or zero dropping is on, not all allowed units may
   actually be used for a given NSDateComponents. Default value is the
   components of the passed-in NSDateComponents object, or years | 
   months | weeks | days | hours | minutes | seconds if passed an
   NSTimeInterval or pair of NSDates.

   Allowed units are:

    NSCalendarUnitYear
    NSCalendarUnitMonth
    NSCalendarUnitWeekOfMonth (used to mean "quantity of weeks")
    NSCalendarUnitDay
    NSCalendarUnitHour
    NSCalendarUnitMinute
    NSCalendarUnitSecond

   Specifying any other NSCalendarUnits will result in an exception.
 */
var allowedUnits: NSCalendarUnit

So:

let formatter = NSDateComponentsFormatter()
formatter.calendar = NSCalendar.currentCalendar()
formatter.allowedUnits = nil
formatter.allowedUnits |= .CalendarUnitYear
formatter.allowedUnits |= .CalendarUnitMonth
formatter.allowedUnits |= .CalendarUnitWeekOfMonth
formatter.allowedUnits |= .CalendarUnitDay
formatter.allowedUnits |= .CalendarUnitHour
formatter.allowedUnits |= .CalendarUnitMinute
formatter.allowedUnits |= .CalendarUnitSecond

let referenceDate = NSDate(timeIntervalSinceReferenceDate: 0)
let intervalDate = NSDate(timeInterval: 214458810, sinceDate: referenceDate)
let string = formatter.stringFromDate(referenceDate, toDate: intervalDate)
// -> "6y 9m 2w 4d 3:53:30"

---

When we omit one of them in the middle:

formatter.allowedUnits = nil
formatter.allowedUnits |= .CalendarUnitYear
formatter.allowedUnits |= .CalendarUnitMonth
formatter.allowedUnits |= .CalendarUnitWeekOfMonth
formatter.allowedUnits |= .CalendarUnitDay
// formatter.allowedUnits |= .CalendarUnitHour
formatter.allowedUnits |= .CalendarUnitMinute
formatter.allowedUnits |= .CalendarUnitSecond

'Specifying positional units with gaps is ambiguous, and therefore unsupported' error :) That is the "gap" means, I think.

This works for me to get rid of the "gaps":

formatter.allowedUnits = .DayCalendarUnit | .HourCalendarUnit | .MinuteCalendarUnit | .SecondCalendarUnit

Not specifying a unit style will cause this in Swift 4 and 5.

let componentsFormatter = DateComponentsFormatter()
componentsFormatter.unitsStyle = .full

Other unit styles: .spellOut, .short, .brief, .abbreviated, .positional