Simple ParseableFormatStyle conformace for TimeInterval, with and without milliseconds.

Can accept string timestamps formatted with hours, minutes, seconds, and, optionally, milliseconds. The parser will handle missing 0-padding

• 0:10:45.333 -> Valid
• 0:2:45.333 -> Valid (0-padding missing OK)
• 20:10:45.2 -> Valid (0 padding on milliseconds missing)
• 20:10:45.200 -> Valid (more than 1 hour digit)
• 0:10:45 -> Valid (no milliseconds)
• 0:10:45.22323 -> Valid (milliseconds go beyond thousandths place)
• 0:10:45:220 -> Valid (millisecond delimiter swapped)

• 10:45.222 -> Invalid (must have hours, even if 0)
• 0:10:45.aaa -> Invalid (non-digit characters)
• 0-10:45.000 -> Invalid (wrong delimiter)
• 1:10:62.222 -> Invalid (too many seconds)

Using showsMilliseconds the client can control if milliseconds are shown or not. All output will be 0-padded and contains exactly 3 places of milliseconds.

• With milliseconds: 2:03:44.232
• Without milliseconds: 2:03:44

I built this to be able to easily move to/from TimeIntervals in SwiftUI TextFields. A simple example below!

struct TimeIntervalTextField: View {
    @Binding var duration: TimeInterval
    
    var body: some View {
        TextField("Duration:", value: $duration, format: .timeInterval(showMilliseconds: true), prompt: nil)
    }
}