Active Days (AD) are an indicator of how long a player has been in the realm, they increment by one (1) each day a player logs in. A player can view theirs at the top of the interface by their name, or by looking near the bottom of their profile.
There are indicators on players to represent their activity days as shown below. In the initial image, the first layer of arrows indicate one (1) AD each, the second layer indicates two (2) AD each, and continuing in the same fashion. The second portrayal of AD comes into effect after a years activity, and replaces the arrow system. The amount of diamonds within the larger diamond reflects the increment of Days - each additional diamond (up to a total of six) indicates another thirty (30) Active Days.
The indicators reflect Active Days up to 539 Days, when the sixth diamond is added.