init(uptimeNanoseconds:)

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.