In JDK 9, the implementation of Clock has been improved to be better
than millisecond in most cases. However, the Javadoc of the "Implementation Note" is now wrong. It says:

"The clock implementation provided here is based on
System.currentTimeMillis(). That method provides little to no
guarantee about the accuracy of the clock. Applications requiring a
more accurate clock must implement this abstract class themselves
using a different external clock, such as an NTP server."

This needs to be updated to match the changes to the implementation.
Suggested replacement text:

"The clock implementation provided here is based on the same
underlying clock as System.currentTimeMillis(), but may have a
precision finer than milliseconds if available. However, little to no
guarantee is provided about the accuracy of the underlying clock. 
Applications requiring a more accurate clock must implement this
abstract class themselves using a different external clock, such
 as an NTP server."