An attempt to demonstrate why java.lang.System.nanoTime is not appropriate to use in making a java.time.Clock.
I reiterate the following from the System.nanoTime documentation:
This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time.
Time is an illusion. Lunchtime doubly so.
-- Douglas Adams, The Hitchhiker's Guide to the Galaxy
See also:
Falsehoods programmers believe about time
Admiral Grace Hopper Explains the Nanosecond