Get time difference between two times in swift 3

The recommended way to do any date math is Calendar and DateComponents

let difference = Calendar.current.dateComponents([.hour, .minute], from: time1, to: time2)
let formattedString = String(format: "%02ld%02ld", difference.hour!, difference.minute!)

The format %02ld adds the padding zero.

If you need a standard format with a colon between hours and minutes DateComponentsFormatter() could be a more convenient way

let formatter = DateComponentsFormatter()
formatter.allowedUnits = [.hour, .minute]
print(formatter.string(from: time1, to: time2)!)

Now you can do it in swift 5 this way,

func getDateDiff(start: Date, end: Date) -> Int  {
    let calendar = Calendar.current
    let dateComponents = calendar.dateComponents([Calendar.Component.second], from: start, to: end)

    let seconds = dateComponents.second
    return Int(seconds!)

To get duration in seconds between two time intervals, this can be used -

let time1 = Date(timeIntervalSince1970: startTime)
let time2 = Date(timeIntervalSince1970: endTime)
let difference = Calendar.current.dateComponents([.second], from: time1, to: time2)
let duration = difference.second

TimeInterval measures seconds, not milliseconds:

let date1 = Date()
let date2 = Date(timeIntervalSinceNow: 12600) // 3:30

let diff = Int(date2.timeIntervalSince1970 - date1.timeIntervalSince1970)

let hours = diff / 3600
let minutes = (diff - hours * 3600) / 60