Standard Trolliversian Calendar

The Standard Trolliversian Calendar (STC) is the calendar used almost everywhere in the Trolliverse. It was invented in 999999999 BCE. The main concept of the calendar was to start at year 999999999 and count down. Eventually, the calendar reached 0, and everyone thought the world would end. However, it didn't and they just started counting up (It also crashed some computers that weren't prepared for the change, similar to Y2K on Earth.) They then decided when the calendar reached 999999999 CE, it would start counting down again. Each of these 999999999 year long periods are called Eras.
In the STC, on year is measured on how long the average Earth-sized planet takes to orbit a medium sized star in the Habitable Zone of the solar system.