How to stop a time UILabel from resizing at every time increment?

Question

I have a stopwatch feature in my app that uses a centered attributed UILabel with a proportionally spaced font to render time. At every time increment the width

Answer 1

For future readers:

This is how to enable monospaced numbers on iOS 9 with San Francisco:

let originalFont = UIFont.systemFontOfSize(17)
let originalFontDescriptor = originalFont.fontDescriptor()

let fontDescriptorFeatureSettings = [
    [
    UIFontFeatureTypeIdentifierKey: kNumberSpacingType,
    UIFontFeatureSelectorIdentifierKey: kMonospacedNumbersSelector
    ]
]

let fontDescriptorAttributes = [UIFontDescriptorFeatureSettingsAttribute: fontDescriptorFeatureSettings]    
let fontDescriptor = originalFontDescriptor.fontDescriptorByAddingAttributes(fontDescriptorAttributes)
let font = UIFont(descriptor: fontDescriptor, size: 0)

Edit:

Also available on GitHub: https://github.com/salutis/swift-font-monospaced-digits