Format time Interval to hour, minute, second format using Swift

format-time-interval-swift.swift

extension TimeInterval {
    func stringFormatted() -> String {
        let interval = Int(self)
        let seconds = interval % 60
        let minutes = (interval / 60) % 60
        let hours = (interval / (60*60)) % 60
        return String(format: "%02d:%02d:%02d", hours, minutes, seconds)
    }
}

// Test
let startTime: TimeInterval = Date.timeIntervalSinceReferenceDate - (59*60 + 120)
let endTime:  TimeInterval = Date.timeIntervalSinceReferenceDate
(endTime - startTime).stringFormatted()