To answer the question of when, you first have to understand that Myos, the world Cairn is set on, isn’t Earth. The time line is their own, but the best way to understand the setting is going to be to compare it to our own past. After all our history has played a large role in inspiring Cairn.
The ancient history of Myos has been completely wiped from the planet. Most people believe there was a time before the darkness, but there is almost no proof. Some ruins seem to pre-date the darkness, but most have been scrapped for their stones or completely destroyed by time and war that none know who or what built them.
The Age of Twilight was a time of survival. No one knows how long the darkness lasted, but there are records written in stone and ancient manuscripts that document the times up to ten centuries or longer in some cases. How these documents survived that amount of time in such a hostile environment is another mystery. For almost everyone that lived in these times life was hard. Food was scarce, most plants would not grow, and monsters roamed the darkness devouring whatever they could find. In this time of darkness it’s believed that several of the great races went extinct, leaving only the myosian and arcole alive for the next age.
The Dawn Age, was when the various people of Myos began to rise above their hardships and take control of their destiny. Why the darkness receded is still unknown, but the days became regular and predictable again. The people began farming, rallied together to protect one another, and populations started to grow bringing the remaining great races back from the brink of extinction. The end of the Dawn Age is marked by the Rift War, when a race of creatures now known as the Slek poured through a portal in the middle of civil war. The surviving Slek were exiled to an almost inhabitable island.
The current age, Age of the High Sun, is a sort of renaissance for the people. Great minds are safe from persecution in most of the provinces and many philosophers, scientists, and engineers have come together to create the most fantastic inventions. This safety comes at a price though. These great inventors are practically prisoners to their home provinces and the secrets behind their great inventions are heavily guarded.
Cairn mixes fantasy with science fiction and applies each part in different combinations to different cultures. Everything that seems magical in our world we try to ground in our own form of reality. A good example of this are the Thermocrats. They are inspired by ancient greeks in theme, but they are masters of engineering. They use alchemical fire, based on the real greek fire, to make fiery swords and flame throwers. They are the faction that says, what if we combine greek fire, Trojan war culture, sci-fi steam engines, and a religious warship of fire. What we got was a pretty good faction, but placing them in a specific time in relation to Earth is impossible as we cherry pick the best pieces of our past and tweak them.
So when is Cairn? It’s 200 HS as far as most are concerned. The Rift War ended 200 years ago and this great age technology is now.