That was sort of my point. They all return the same thing. UNION takes out the duplicate rows. The answers pertain more to UNION than to the date time issue.
It's one of those things when taking a test. Ever have one of those where you have a wordy question that runs nine paragraphs, defines 40 some-odd variables, and is confusing as anything? Then you look at the multiple answers (of which there are at least 4 dozen) only to see that the 11th from the bottom is: "This test is being taken in room 314A." Your instructions were to choose the best answer and that one is it!