I tried overcast (9/21/18) & it was not accepted. I'm not sure if it's a acceptable translation for dark, but it seems more natural to me. I'll report it & see what happens
Just because it is occasionally said does not make it correct usage. Duo should be teaching correct usage, not regional colloquialisms. "Day" is defined as the hours in which there is sunlight, "night" is time in which it is dark.
No, it shouldn't because that doesn't make sense in English. Today is overcast, or today is cloudy, but if it's dark, it's night.
"It's dark today" is not a sentence that makes sense in English. It is overcast, it is cloudy - those make sense and are common usage. Duo has an obligation to teach in a way that makes sense in either language. Maybe in Spanish "Está oscuro hoy" would be correct usage, but their translation is not common usage - generally if it's dark, it's night, not day - therefore they should accept translations that would be commonly used in English.
So if I say It is dark, the listener won't understand to translate the words as it's , Can't see why wrong if not a contraction
Some translations are exact and some go by meaning. The exact translation is 'dark'. In the right situation such an outdoor photo shoot, we could more likely imagine an English speaker saying "It's dark today.," even though cloudy or overcast are more common.