Translation:Nine days is long.
In written form (~는다) adjectives are left unchanged in the dictionary form. So the conjugation of, for example, 먹다, 가다 and 하다 is 먹는다, 간다 and 한다, but adjectives such as 길다, 쉽다 and 노랗다 are not conjugated, but left as 길다, 쉽다, 노랗다. Therefore it would be a mistake to write 길다 as something like "길는다" or "긴다".
What does the Korean actually mean / imply?
- Nine days are long (but all other days are short or normal length)
- Nine days is a long period of time
If 1 well that's a bit odd but the Korean and English say the same thing and we're used to odd sentences here. If 2 then the English is wrong. If 3 then I guess the English is at least trying to be equally ambiguous I suppose.