I reported the same thing. "Sino" is shown to translate as "rather" as well as "but", so unless it's a grammatical thing, it should be accepted.
I thought "son" = "they are" and "es" = "it is." (Él/ella/usted es ... and ellos/ellas/ustedes son ...)
Because "son" is the third person equivalent of "is", so just as "is not hours, but days" makes no sense because there is no noun referent, the translation is "It is" or "It's"
But you don't really say that in English, there is still the implied "it is" because this only makes sense as a response when the subject is already identified. To translate the sentence given "It's/it is" is needed because "son" is the verb.
I hope this helps a little in understanding the phrase..
I took it to mean something like what was in the movie 2012 or the movie The Day After Tomorrow
I calculated the time wrong..
We don't have:
" years, but days " " days, but hours " " hours, but minutes"
So, with that thought going ... I just reversed it..
It's not [calculated] in hours, but [ it's calculated] in days.
It is not hours, but days.