+ /// - Discussion: This clock is the same as the value returned by
+ /// `mach_absolute_time` when converted into nanoseconds.
+ /// On some platforms, the nanosecond value is rounded up to a
+ /// multiple of the Mach timebase, using the conversion factors
+ /// returned by `mach_timebase_info()`. The nanosecond equivalent
+ /// of the rounded result can be obtained by reading the
+ /// `uptimeNanoseconds` property.
+ /// Note that `DispatchTime(uptimeNanoseconds: 0)` is
+ /// equivalent to `DispatchTime.now()`, that is, its value
+ /// represents the number of nanoseconds since boot (excluding
+ /// system sleep time), not zero nanoseconds since boot.