On Earth, we have several cyclic, natural processes that we use to measure the passage of time: the weather (seasons), the stars (seasonal constellations), the length of days (the declination of the sun above the horizon), and the tides and phases of the moon. However, all of these (except for the last one) are strongly influenced by the tilt of the earth.
In the Game of Thrones universe, the seasons are very long, but moreover, their duration seems to be very hard to predict (compared to our ability to predict future celestial events even 2000 years before they happen). This would suggest that the planet of GoT precesses with a period close to that of its solar orbit but with large-scale chaotic behavior.
As a result, the best bet that an astronomer in Westeros would have for plotting the passage of years would be the phases on the moon. It might also be possibly to use seasonal constellations as well, but either way, there would be no natural time scale for one year as there is on Earth (namely, the seasons and our biological and cultural responses to them). It begs the question:
What defines a year in Westeros? How do people know when one has ended and the next begins?
