I use JodaTime#DateTime, and I need to mock its behavior. Since it is not possible to directly mock JodaTime#DateTime, I create an interface of it<
I think you're definitely on the right path. Creating the Clock interface for mocking is definitely a good idea.
One thing I don't see in your code: injecting the mocked clock into the printProcessor. After creating the mock, I think you need something along the lines of:
printProcessor.setClock(clock)
(this goes before you call getPrintJobName. This setter should set the jodaTime property in your PrintProcessor class)
I'm used to EasyMock, so I might be wrong, but I'm pretty sure you will also need to set expectations for the getFourPM_EST and getSevenPM_EST calls.