Swift 2 function for human-readable time interval

relativeDateInterval.swift

/// returns optional relative date interval string like "2d"
/// depending on the unitsstyle, see docs at http://apple.co/1ox2sOX
/// inject an existing NSDateComponentsFormatter() for performance
func relativeDateInterval(date: NSDate, 
  unitsStyle: NSDateComponentsFormatterUnitsStyle, 
  formatter: NSDateComponentsFormatter) -> String? {
    // inspired by top answer at http://bit.ly/1TzMQqV
    let formatter = NSDateComponentsFormatter()
    formatter.unitsStyle = unitsStyle //.Abbreviated, .Full, ...
    formatter.includesApproximationPhrase = false
    formatter.includesTimeRemainingPhrase = false
    formatter.allowedUnits = [.Year, .Month, .WeekOfMonth, .Day, .Hour, .Minute, .Second]
    formatter.maximumUnitCount = 1
    
    return formatter.stringFromDate(date, toDate: NSDate())
}
    
    
//////////////   
// Use example
let formatter = NSDateComponentsFormatter()
let date = NSDate() // change that to a date in the past or it will display "0s"
let str = relativeDateInterval(date, unitsStyle: .Abbreviated, formatter: formatter)
print(str)