Eras in the world of Deluna specifically refer to 1000 year periods, after which they begin from scratch. For example, the year can be the 6th year of the 12th Era, which basically means 12006 years since the Eras were established.
The creation of the Eras became useful as time was not very well documented in older times, much info was lost to greatwars and hiding in great fortresses. When humans awoke from their icy grave, they thought they were the only ones there, saying they had begun their first year. Humourosly, elven scholars based a new dating system for their next middaara, which for them is 12000 years starting with the humans' arrival into the world. However after the beggining of rise of humans, it became useful with the many races of the planet. This became the Common Eras using DR to indicate common Era. Anything before was known as the Elder Eras, using AR to describe such times.