Years in Westeros

It occurred to me: The seasons in A Song of Ice and Fire are many years long, right? In the show, Old Nan said that men were born, lived, and died all without seeing the sun (presumably in winter, in the north). But when the characters talk about how long the seasons are, they use the word “years.” (At least, I think they do. I would have to comb through the books to be sure.) But since for us a year is defined as one revolution around the sun, or one cycle of seasons, how do the people in Westeros know how long a “year” is, since it would be some fraction of the length of their seasons? Why would they even have a concept for a fraction of time shorter than a season?

And how does their crop rotations work? Do they have to harvest and store food all through summer and fall because no food grows for years during the winter?

These kinds of things go through my head when I’m worldbuilding my own worlds. Things like, “Why are there 12 seasons months in a year? Why aren’t there 10? Or 15? There’s no astrological equivalent for months so somebody must have just made it up out of thin air. How would 5,000 years of civilization be different if that person decided to use 20 months? Or no months at all? Oh yeah, why did that guy pick 7 days to put into a week? Why did we need a week at all?”

Hrm. Maybe when they say the word “year” they mean a unit of measurement similar to our word for “month.”