Less than a full year actually passes in each novel.The child actors in the TV series, however, still age at a normal rate during production, so in order to keep consistent, the TV series generally follows the rule that one TV season equals one year in the storyline.On the average, it seems that one season can last for about two to three years or so (the full four season cycle therefore taking about a decade).There are hints that the seasons may not always have been this way: characters still define "a year" as a twelve month period, not a full cycle of summer to winter.At the time of the novels, Westeros has been using a calendar system based on the year of Aegon's Landing, which occurred three centuries before.As explained above, calling it "Aegon's Landing" (AL) is somewhat anachronistic given that the "landing" happened at the beginning of the conquest but the calendar system only begins two years later, at the end of the conquest - more recent in-universe historical texts have been shifting to the alternate name "After Conquest" (AC).Her Unsullied trapped at Casterly Rock and Highgarden sacked with the Tyrells dead, Daenerys Targaryen leads the Dothraki atop Drogon against the combined Lannister and Tarly army and destroys their loot train at the Battle of the Goldroad.Bran Stark, having returned to Winterfell, witnesses the marriage of Rhaegar Targaryen and Lyanna Stark and learns that Jon Snow, whose true name is Aegon Targaryen, is the rightful heir to the Iron Throne.
Still, all legends and oral histories may have some kernel of truth behind them. Martin himself, is that as the saying goes, history tends to be written by the victors.
Not every culture throughout real life history has measured the first hour of a given day starting at midnight (the exact opposite of noon); some start at sunrise, others at sunset.
It isn't clear at what hour one day officially becomes the next in Westeros (though given that they are an agrarian society, they probably measure by each sunrise).
The Northerners, descended from the First Men who were never conquered by the Andals, have a decidedly negative view of the Andal invasions.
The dating system is based on the Targaryen Conquest.