Although "is over" looks like a present simple tense it actually means "has finished" which indicates present-state reached in the past. In Italian this would be "è finito" (or possibly "ha finito"), whereas in this sentence we have "finisce" which is present tense and thus should be translated as "finishes/ends" or possibly "is finishing" / "is ending"
I think part of a good language-learning technique is to learn whole phrases and a good and natural way to do it is when you are able to imagine a situation in which you can use that phrase. It's true that translating a phrase like "the decade ends today" helps us practice the words we've learnt, but Duolingo, i would argue, is wasting an great opportunity for us to better our language capacity by insisting on using such useless phrases where imagination is not provoked. Yeah but still, Duolingo is my favourite language-learning app there is! ;)