Swift supports remainder calculations on floating-point numbers, so we can use % 1.

var ms = Int((interval % 1) * 1000)

as in:

func stringFromTimeInterval(interval: TimeInterval) -> NSString {


let ti = NSInteger(interval)


let ms = Int((interval % 1) * 1000)


let seconds = ti % 60
let minutes = (ti / 60) % 60
let hours = (ti / 3600)


return NSString(format: "%0.2d:%0.2d:%0.2d.%0.3d",hours,minutes,seconds,ms)
}

result:

stringFromTimeInterval(12345.67)                   "03:25:45.670"

Swift 4:

extension TimeInterval{


func stringFromTimeInterval() -> String {


let time = NSInteger(self)


let ms = Int((self.truncatingRemainder(dividingBy: 1)) * 1000)
let seconds = time % 60
let minutes = (time / 60) % 60
let hours = (time / 3600)


return String(format: "%0.2d:%0.2d:%0.2d.%0.3d",hours,minutes,seconds,ms)


}
}

Use:

self.timeLabel.text = player.duration.stringFromTimeInterval()

Equivalent in Objective-C, based on the @matthias-bauch answer.

+ (NSString *)stringFromTimeInterval:(NSTimeInterval)timeInterval
{
NSInteger interval = timeInterval;
NSInteger ms = (fmod(timeInterval, 1) * 1000);
long seconds = interval % 60;
long minutes = (interval / 60) % 60;
long hours = (interval / 3600);


return [NSString stringWithFormat:@"%0.2ld:%0.2ld:%0.2ld,%0.3ld", hours, minutes, seconds, (long)ms];
}

for convert hour and minutes to seconds in swift 2.0:

///RETORNA TOTAL DE SEGUNDOS DE HORA:MINUTOS
func horasMinutosToSeconds (HoraMinutos:String) -> Int {


let formatar = NSDateFormatter()
let calendar = NSCalendar.currentCalendar()
formatar.locale = NSLocale.currentLocale()
formatar.dateFormat = "HH:mm"


let Inicio = formatar.dateFromString(HoraMinutos)
let comp = calendar.components([NSCalendarUnit.Hour, NSCalendarUnit.Minute], fromDate: Inicio!)


let hora = comp.hour
let minute = comp.minute


let hours = hora*3600
let minuts = minute*60


let totseconds = hours+minuts


return totseconds
}

converted into an swift 2 extension + variable format :

extension NSTimeInterval {


func timeIntervalAsString(format format : String = "hh:mm:ss:sss") -> String {
let ms      = Int((self % 1) * 1000)
let asInt   = NSInteger(self)
let s = asInt % 60
let m = (asInt / 60) % 60
let h = (asInt / 3600)


var value = format
value = value.replace("hh",  replacement: String(format: "%0.2d", h))
value = value.replace("mm",  replacement: String(format: "%0.2d", m))
value = value.replace("sss", replacement: String(format: "%0.3d", ms))
value = value.replace("ss",  replacement: String(format: "%0.2d", s))
return value
}


}


extension String {
/**
Replaces all occurances from string with replacement
*/
public func replace(string:String, replacement:String) -> String {
return self.stringByReplacingOccurrencesOfString(string, withString: replacement, options: NSStringCompareOptions.LiteralSearch, range: nil)
}


}

swift 3 version of @hixField answer, now with days and handling previous dates:

extension TimeInterval {
func timeIntervalAsString(_ format : String = "dd days, hh hours, mm minutes, ss seconds, sss ms") -> String {
var asInt   = NSInteger(self)
let ago = (asInt < 0)
if (ago) {
asInt = -asInt
}
let ms = Int(self.truncatingRemainder(dividingBy: 1) * (ago ? -1000 : 1000))
let s = asInt % 60
let m = (asInt / 60) % 60
let h = ((asInt / 3600))%24
let d = (asInt / 86400)


var value = format
value = value.replacingOccurrences(of: "hh", with: String(format: "%0.2d", h))
value = value.replacingOccurrences(of: "mm",  with: String(format: "%0.2d", m))
value = value.replacingOccurrences(of: "sss", with: String(format: "%0.3d", ms))
value = value.replacingOccurrences(of: "ss",  with: String(format: "%0.2d", s))
value = value.replacingOccurrences(of: "dd",  with: String(format: "%d", d))
if (ago) {
value += " ago"
}
return value
}


}

SWIFT 3 Extension

I think this way is a easier to see where each piece comes from so you can more easily modify it to your needs

extension TimeInterval {
private var milliseconds: Int {
return Int((truncatingRemainder(dividingBy: 1)) * 1000)
}


private var seconds: Int {
return Int(self) % 60
}


private var minutes: Int {
return (Int(self) / 60 ) % 60
}


private var hours: Int {
return Int(self) / 3600
}


var stringTime: String {
if hours != 0 {
return "\(hours)h \(minutes)m \(seconds)s"
} else if minutes != 0 {
return "\(minutes)m \(seconds)s"
} else if milliseconds != 0 {
return "\(seconds)s \(milliseconds)ms"
} else {
return "\(seconds)s"
}
}
}

Swift 3 solution for iOS 8+, macOS 10.10+ if the zero-padding of the hours doesn't matter:

func stringFromTime(interval: TimeInterval) -> String {
let ms = Int(interval.truncatingRemainder(dividingBy: 1) * 1000)
let formatter = DateComponentsFormatter()
formatter.allowedUnits = [.hour, .minute, .second]
return formatter.string(from: interval)! + ".\(ms)"
}


print(stringFromTime(interval: 12345.67)) // "3:25:45.670"

Swift 4, without using the .remainder (which returns wrong values):

func stringFromTimeInterval(interval: Double) -> NSString {


let hours = (Int(interval) / 3600)
let minutes = Int(interval / 60) - Int(hours * 60)
let seconds = Int(interval) - (Int(interval / 60) * 60)


return NSString(format: "%0.2d:%0.2d:%0.2d",hours,minutes,seconds)
}

Swift 4:

extension TimeInterval{


func stringFromTimeInterval() -> String {


let time = NSInteger(self)


let ms = Int((self.truncatingRemainder(dividingBy: 1)) * 1000)
let seconds = time % 60
let minutes = (time / 60) % 60
let hours = (time / 3600)


return String(format: "%0.2d:%0.2d:%0.2d.%0.3d",hours,minutes,seconds,ms)


}
}

Use:

self.timeLabel.text = player.duration.stringFromTimeInterval()

I think most of those answers are outdated, you should always use DateComponentsFormatter if you want to display a string representing a time interval, because it will handle padding and localization for you.

Swift 4 Extension - with nanoseconds precision

import Foundation


extension TimeInterval {


func toReadableString() -> String {


// Nanoseconds
let ns = Int((self.truncatingRemainder(dividingBy: 1)) * 1000000000) % 1000
// Microseconds
let us = Int((self.truncatingRemainder(dividingBy: 1)) * 1000000) % 1000
// Milliseconds
let ms = Int((self.truncatingRemainder(dividingBy: 1)) * 1000)
// Seconds
let s = Int(self) % 60
// Minutes
let mn = (Int(self) / 60) % 60
// Hours
let hr = (Int(self) / 3600)


var readableStr = ""
if hr != 0 {
readableStr += String(format: "%0.2dhr ", hr)
}
if mn != 0 {
readableStr += String(format: "%0.2dmn ", mn)
}
if s != 0 {
readableStr += String(format: "%0.2ds ", s)
}
if ms != 0 {
readableStr += String(format: "%0.3dms ", ms)
}
if us != 0 {
readableStr += String(format: "%0.3dus ", us)
}
if ns != 0 {
readableStr += String(format: "%0.3dns", ns)
}


return readableStr
}
}

Swift 4 (with Range check ~ without Crashes)

import Foundation


extension TimeInterval {


var stringValue: String {
guard self > 0 && self < Double.infinity else {
return "unknown"
}
let time = NSInteger(self)


let ms = Int((self.truncatingRemainder(dividingBy: 1)) * 1000)
let seconds = time % 60
let minutes = (time / 60) % 60
let hours = (time / 3600)


return String(format: "%0.2d:%0.2d:%0.2d.%0.3d", hours, minutes, seconds, ms)


}
}

Here is slightly improved version of @maslovsa's, with Precision input param:

import Foundation


extension TimeInterval {


enum Precision {
case hours, minutes, seconds, milliseconds
}


func toString(precision: Precision) -> String? {
guard self > 0 && self < Double.infinity else {
assertionFailure("wrong value")
return nil
}


let time = NSInteger(self)


let ms = Int((self.truncatingRemainder(dividingBy: 1)) * 1000)
let seconds = time % 60
let minutes = (time / 60) % 60
let hours = (time / 3600)


switch precision {
case .hours:
return String(format: "%0.2d", hours)
case .minutes:
return String(format: "%0.2d:%0.2d", hours, minutes)
case .seconds:
return String(format: "%0.2d:%0.2d:%0.2d", hours, minutes, seconds)
case .milliseconds:
return String(format: "%0.2d:%0.2d:%0.2d.%0.3d", hours, minutes, seconds, ms)
}
}
}

and usage:

let time: TimeInterval = (60 * 60 * 8) + 60 * 24.18
let hours = time.toString(precision: .hours) // 08
let minutes = time.toString(precision: .minutes) // 08:24
let seconds = time.toString(precision: .seconds) // 08:24:10
let milliseconds = time.toString(precision: .milliseconds) // 08:24:10.799

Swift 5. No ms and some conditional formatting (i.e. don't display hours if there are 0 hours).

extension TimeInterval{


func stringFromTimeInterval() -> String {


let time = NSInteger(self)


let seconds = time % 60
let minutes = (time / 60) % 60
let hours = (time / 3600)


var formatString = ""
if hours == 0 {
if(minutes < 10) {
formatString = "%2d:%0.2d"
}else {
formatString = "%0.2d:%0.2d"
}
return String(format: formatString,minutes,seconds)
}else {
formatString = "%2d:%0.2d:%0.2d"
return String(format: formatString,hours,minutes,seconds)
}
}
}

You can use Measurement and UnitDuration to convert a TimeInterval value into any duration unit. To see milliseconds in the result you need UnitDuration.milliseconds that requires iOS 13.0, tvOS 13.0, watchOS 6.0 or macOS 10.15. I put all actions that should be done in func convertDurationUnitValueToOtherUnits(durationValue:durationUnit:smallestUnitDuration:) (Swift 5.1.3/Xcode 11.3.1):

import Foundation


@available(iOS 10.0, tvOS 10.0, watchOS 3.0, macOS 10.12, *)
func convert<MeasurementType: BinaryInteger>(
measurementValue: Double, unitDuration: UnitDuration, smallestUnitDuration: UnitDuration
) -> (MeasurementType, Double) {
let measurementSmallest = Measurement(
value: measurementValue,
unit: smallestUnitDuration
)
let measurementSmallestValue = MeasurementType(measurementSmallest.converted(to: unitDuration).value)
let measurementCurrentUnit = Measurement(
value: Double(measurementSmallestValue),
unit: unitDuration
)
let currentUnitCount = measurementCurrentUnit.converted(to: smallestUnitDuration).value
return (measurementSmallestValue, measurementValue - currentUnitCount)
}


@available(iOS 10.0, tvOS 10.0, watchOS 3.0, macOS 10.12, *)
func convertDurationUnitValueToOtherUnits<MeasurementType: BinaryInteger>(
durationValue: Double,
durationUnit: UnitDuration,
smallestUnitDuration: UnitDuration
) -> [MeasurementType] {
let basicDurationUnits: [UnitDuration] = [.hours, .minutes, .seconds]
let additionalDurationUnits: [UnitDuration]
if #available(iOS 13.0, tvOS 13.0, watchOS 6.0, macOS 10.15, *) {
additionalDurationUnits = [.milliseconds, .microseconds, .nanoseconds, .picoseconds]
} else {
additionalDurationUnits = []
}
let allDurationUnits = basicDurationUnits + additionalDurationUnits
return sequence(
first: (
convert(
measurementValue: Measurement(
value: durationValue,
unit: durationUnit
).converted(to: smallestUnitDuration).value,
unitDuration: allDurationUnits[0],
smallestUnitDuration: smallestUnitDuration
),
0
)
) {
if allDurationUnits[$0.1] == smallestUnitDuration || allDurationUnits.count <= $0.1 + 1 {
return nil
} else {
return (
convert(
measurementValue: $0.0.1,
unitDuration: allDurationUnits[$0.1 + 1],
smallestUnitDuration: smallestUnitDuration
),
$0.1 + 1
)
}
}.compactMap { $0.0.0 }
}

This is how you can call it:

let intervalToConvert: TimeInterval = 12345.67
let result: [Int] = convertDurationUnitValueToOtherUnits(
durationValue: intervalToConvert,
durationUnit: .seconds,
smallestUnitDuration: .milliseconds
)
print("\(result[0]) hours, \(result[1]) minutes, \(result[2]) seconds, \(result[3]) milliseconds") // 3 hours, 25 minutes, 45 seconds, 670 milliseconds

As you can see I did not use numeric constants like 60 and 1000 to get the result.