date - How to convert TimeInterval into Minutes, Seconds and Milliseconds in Swift

Question

I am writing a racing app and want to convert a large number of milliseconds into minutes:seconds.milliseconds (for example, 1:32.361). At the moment I just do it through maths (minutes / 60000), then get remainder and divide further, etc., but i find that it is off every now and then by 1 millisecond. I know this might not sound like a big deal, but I tried to find the average of 1:32.004 and 1:32.004 and it returned 1:32.003, which means it gets put in incorrectly and the rest of the app can't deal with it.

Is there a way to use NSDate or NSDateFormatter or something like that to format the milliseconds into MM:ss.mmm?

I have looked at these sources but not found the answer:

How to convert milliseconds into minutes?
Convert seconds into minutes and seconds
Convert milliseconds to minutes
http://www.codingexplorer.com/swiftly-getting-human-readable-date-nsdateformatter/

Answer 1

You can create a TimeInterval extension to format your elapsed time as follow:

Xcode 11.4 ? Swift 5.2 or later

extension TimeInterval {
    var hourMinuteSecondMS: String {
        String(format:"%d:%02d:%02d.%03d", hour, minute, second, millisecond)
    }
    var minuteSecondMS: String {
        String(format:"%d:%02d.%03d", minute, second, millisecond)
    }
    var hour: Int {
        Int((self/3600).truncatingRemainder(dividingBy: 3600))
    }
    var minute: Int {
        Int((self/60).truncatingRemainder(dividingBy: 60))
    }
    var second: Int {
        Int(truncatingRemainder(dividingBy: 60))
    }
    var millisecond: Int {
        Int((self*1000).truncatingRemainder(dividingBy: 1000))
    }
}

---

extension Int {
    var msToSeconds: Double { Double(self) / 1000 }
}

---

let seconds = 131.531   // 131.531
let time = seconds.minuteSecondMS  //  "2:11.531"
let millisecond = seconds.millisecond    // 531

let ms = 1111
let sec = ms.msToSeconds.minuteSecondMS   // "0:01.111"

---

Date().description(with: .current)        // "Wednesday, October 21, 2020 at 5:44:51 PM Brasilia Standard Time"
let startOfDay = Calendar.current.startOfDay(for: Date())
let secondsFromStartOfDay = Date().timeIntervalSinceReferenceDate - startOfDay.timeIntervalSinceReferenceDate
secondsFromStartOfDay.hourMinuteSecondMS  // "17:44:51.705"