According to their changelog[0], Clock was added to the standard library with Swift 5.7, which shipped in 2022, at the same time as iOS 16. It looks like static

According to their changelog[0], Clock was added to the standard library with Swift 5.7, which shipped in 2022, at the same time as iOS 16. It looks like static linking by default was approved[1] but development stalled[2]. I expect that it's as simple as that: It's supported on iOS 16+ because it's dynamically linked by default, against a system-wide version of the standard library. You can probably try to statically link newer versions on old OS versions, or maybe ship a newer version of the standard library and dynamically link against that, but I have no idea how well those paths are supported. 0. https://github.com/apple/swift/blob/main/CHANGELOG.md https://github.com/apple/swift/blob/main/CHANGELOG.md 1. https://github.com/apple/swift-evolution/blob/main/proposals/0342-static-link-runtime-libraries-by-default-on-supported-platforms.md https://github.com/apple/swift-evolution/blob/main/proposals... 2. https://github.com/apple/swift-package-manager/pull/3905 https://github.com/apple/swift-package-manager/pull/3905