A DESCRIPTION OF THE PROBLEM :
java.time.Duration::plusNanos and a lot of java.time methods have a similar part in their description:
"Returns a copy of this duration with the specified duration in nanoseconds added."
It is misleading, because such methods neither change nor even create a copy, but generally have a chain of new object constructions and nothing common between argument and result.
Better description is possible. An example for this particular case:
Returns a Duration object with added specified nanoseconds to original value.