dispatch_time(3) on macOS

The text 8fb8f6e863ad, as macOS 10.12.6, macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

Calculate temporal milestones

dispatch_time(3) on macOS now