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.