BOTH campaigns that start this weekend — Rise of the Runelords and Shattered Star — will begin on the 20th day of Rova, 4712 AR. What on Golarion does this mean, you may wonder?
Days and months on Golarion are very similar to how we measure time here on Earth. Oathday is similar to Thursday, and Rova is similar to September. “AR” stands for Absalom Reckoning” — Year One was when the god Aroden raised the Starstone from the Inner Sea, thus establishing Absalom, the City at the Center of the World.
For a full list of days and months, see the Pathfinder Wiki.