If you want to use DI, in java 8 you could inject a java.time.Clock instance in the constructor and provide a fixed instance at the required time in your test e.g.
Instant testNow = ...
User u = new User(Clock.fixed(testNow, ZoneOffset.UTC));
u.sayCurrentTime();
although it would be better design to have sayCurrentTime take a date parameter instead of depending on an external dependency.
In my experience the need to mock out individual methods like this is an indication that the code is badly structured in the first place. The time source is effectively a global variable so in this example you'd want to pass the time as a parameter to `sayCurrentTime` and avoid the need to mock anything in the first place. A lot of C#/java codebases do seem to make excessive use of mocks and DI in this way though.
