"O dia está passando rápido demais."
Does that mean you you think it should be accepted? I have reported several English sentences which are badly worded or are just plain gibberish. There is no problem with their English here, the problem is whether my sentence translates the Portuguese and when Duolingo rejects my answer I can only sit and wonder why. When I compare the two English sentences mine is the stronger sentiment so they are not exact equivalents and my doubts remain.
As a foreigner, I would say it translates well. I can't think of a way to rebuild the sentence, in Portuguese, to make it "stronger". The word demais is already the strongest I can get. The only thing I could do to make the sentence sound stronger would be to "vocally" emphasize the word demais.
It's the same thing in French. In Portuguese we would translate "trop" and "beaucoup trop" as "demais", since it sounds very bizarre to say "muito demais".