Board Thread:Questions and Answers/@comment-97.76.56.166-20160516201428/@comment-5014364-20160517155118

"One Year" for them is 12 "moon-turns" (lunar months).

Many book readers think the fact that they still measure "a year" is a hint that the seasons weren't always the same.

The major factor, though, is that the maesters and other scholars do study astronomy, and the entire planet does move around the sun in the time of one "year" -- it's the time it takes for the constellations in the night sky to reset.