Dating in the dark rickard
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).Each "day" apparently consists of a 24 hour period - simply so that Martin would not confuse readers when he referred to a certain amount of hours in the narrative.People in Westeros apparently just apply colloquial names to each hour of the day, i.e., the "hour of the wolf" is the darkest time in the middle of the night.Other characters are older (Eddard Stark and Robert Baratheon are ten years older than their book counterparts) or younger (Ser Vardis Egen is decades younger than in the book, while Theon is two years younger), though for the most part this has no bearing on the timeline.In the book chronology, roughly two years pass between the beginning of A Game of Thrones and the end of the third novel, A Storm of Swords.
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.
Even more simple "cultural traditions" and oral histories have much to say on the subject but no hard evidence.
Some of these oral traditions are known to be simply inaccurate: the Dothraki believe that the first man came into being one thousand years ago, when even the written histories of other continuous civilizations stretch back five to six thousand years.
Months are the same as in real-life, roughly a thirty day period. Apparently Westeros doesn't actually have specific names for each month/moon-turn - given that even after five novels a month name has never been mentioned, and the actual month names stem from real-life history (i.e., July and August were named after Roman Emperors).