History of Kashyyyk
The history of the planet Kashyyyk can be a bit confusing for off-worlders to understand. For many years, Wookiee history was passed down through a tradition of oral history. This practice was continued much longer than in other cultures, which makes it difficult to compile a comprehensive history of Kashyyyk as has been done for other planets.
Confusion about Wookiee history is further complicated by the way Wookiees have named historical eras. Because trees are central to Wookiee culture, the traditional way to group years into eras has been to describe them as "Rings", like the growth rings of trees. Wookiees always refer to the era using the plural, since they consider the term to cover all the years of the era.
Historic "Rings" are usually given a name that describes it's significance to Wookiee history. Typically, the beginning or end of a period of "Rings" is associated with a specific event. However, it is not uncommon for only the beginning or end event to be clearly defined. It is also quite common for historic "Rings" to overlap with each other.
For this reason, it is usually just better to view Wookiee history as the Wookiees do, instead of trying to convert it into any sort of Galactic measure of time. Several examples are listed below.
Rings of Exploration
This is the Wookiee name for the period in which they first gained access to space travel. During this period the Wookiees explored the system around Kashyyyk and colonized the moons of Kashyyyk. It was also during this time that they made first contact with the Trandoshan species which was native to Trandosha, another planet in their solar system.
Rings of Trandoshan Aggression
The Rings of Trandoshan Aggression is how the Wookiees describe the periods of conflict with the Trandoshan species. This term can be used to describe all years included between first contact with the Trandoshans through the the current Galactic Civil War.
It is also commonly used for specific periods of conflict. In this sense, Wookiees will usually indicate which period by including the name of a battle or event from the smaller period. For example, the term "Rings of Trandoshan Agression -- Slaver Years" would refer to the specific years since Trandoshan slavers began operating on Kashyyyk.