stopwatch with milliseconds in SwiftUI - swiftui

Question

I am trying to make a stopwatch with milliseconds. For this, I have copied the code from Fini Eines (https://medium.com/geekculture/build-a-stopwatch-in-just-3-steps-using-swiftui-778c327d214b)
However, I am looking for a stopwatch with milliseconds. Therefore I have added my own var's and parameters. My goal is only to have a stopwatch, which consistently shows hours, minutes, seconds, milliseconds. Features for laps are not necessary.
It just doesn't seem to work. Am I forgetting something?
This is my code:
struct Stopwatch: View {
/// Current progress time expresed in seconds
#State private var progressTime = 0
#State private var isRunning = false
/// Computed properties to get the progressTime in hh:mm:ss format
var hours: Int {
progressTime / 3600
}
var minutes: Int {
(progressTime % 3600) / 60
}
var seconds: Int {
progressTime % 60
}
var milliseconds: Int {
(progressTime % 60) / 1000
}
/// Increase progressTime each second
#State private var timer: Timer?
var body: some View {
VStack {
HStack(spacing: 10) {
StopwatchUnit(timeUnit: hours, timeUnitText: "", color: .pink)
Text(":")
.font(.system(size: 48))
.foregroundColor(.white)
.offset(y: -18)
StopwatchUnit(timeUnit: minutes, timeUnitText: "", color: .blue)
Text(":")
.font(.system(size: 48))
.foregroundColor(.white)
.offset(y: -18)
StopwatchUnit(timeUnit: seconds, timeUnitText: "", color: .green)
Text(",")
.font(.system(size: 48))
.foregroundColor(.white)
.offset(y: -18)
StopwatchUnit(timeUnit: milliseconds, timeUnitText: "", color: .green)
}
HStack {
Button(action: {
if isRunning{
timer?.invalidate()
} else {
timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true, block: { _ in
progressTime += 0
})
}
isRunning.toggle()
})
}
}
}
struct StopwatchUnit: View {
var timeUnit: Int
var timeUnitText: String
var color: Color
/// Time unit expressed as String.
/// - Includes "0" as prefix if this is less than 10.
var timeUnitStr: String {
let timeUnitStr = String(timeUnit)
return timeUnit < 10 ? "0" + timeUnitStr : timeUnitStr
}
var body: some View {
VStack {
ZStack {
HStack(spacing: 2) {
Text(timeUnitStr.substring(index: 0))
.font(.system(size: 48))
.frame(width: 28)
Text(timeUnitStr.substring(index: 1))
.font(.system(size: 48))
.frame(width: 28)
}
}
Text(timeUnitText)
.font(.system(size: 16))
}
.foregroundColor(.white)
}
}
extension String {
func substring(index: Int) -> String {
let arrayString = Array(self)
return String(arrayString[index])
}
}
Thanks for any help!