To make an important addition to the answer of Jon Skeet, Java 9 is supposed to deliver a clock in improved precision - see the bug log. Background: On many operating systems (especially Linux), there are better clocks available.
The Java SE 8 specification for java.time.Clock states that "The
system factory methods provide clocks based on the best available
system clock. This may use System.currentTimeMillis(), or a higher
resolution clock if one is available.". In JDK 8 the implementation
of the clock returned was based on System.currentTimeMillis(), and
thus has only a millisecond resolution. In JDK 9, the implementation
is based on the underlying native clock that
System.currentTimeMillis() is using, providing the maximum resolution
available from that clock. On most systems this can be microseconds,
or sometimes even tenth of microseconds.
An application making the assumption that the clock returned by these
system factory methods will always have milliseconds precision and
actively depends on it, may therefore need to be updated in order to
take into account the possibility of a greater resolution, as was
stated in the API documentation.
It should also be noted the (exotic) fact that second precision will not exist near leap seconds - not even in Java 9.
