Mocks are useful for providing state where it's queried from external objects. They are also great to verify that the intended external side effects of an action occur. verify isn't / unit tests aren't the right mechanism for forcing a particular implementation (which is what your example test does - why do you care if getHour is called, it doesn't update state in another part of the system?) Your tests should almost never never say "this method on this mock will be called X number of times with this result", unless it's something that you actually care about (in which case it should be the entire subject of the test).
The above test is much easier without mocks. You construct a date, pass it in, then check that the result is equal to the input plus 10 days. That's all you care about, so that's all you test.
Verify is useful if you have to update external state. If, in your contrived example, for whatever reason, you needed to keep a tally of all of the times that due dates were ever calculated and that was stored somewhere else, you might have a test that mocks your CalculationCounterClass and verifies that a method (addCalculatedDueDate or whatever) gets called on there...
Good, simple (and IMO brittle) tests that confirm what you care about are useful. Forcing a particular implementation through your tests is a drain.
The above test is much easier without mocks. You construct a date, pass it in, then check that the result is equal to the input plus 10 days. That's all you care about, so that's all you test.
Verify is useful if you have to update external state. If, in your contrived example, for whatever reason, you needed to keep a tally of all of the times that due dates were ever calculated and that was stored somewhere else, you might have a test that mocks your CalculationCounterClass and verifies that a method (addCalculatedDueDate or whatever) gets called on there...
Good, simple (and IMO brittle) tests that confirm what you care about are useful. Forcing a particular implementation through your tests is a drain.