Well from what we've been able to tell this isn't Earth so whatever year we pick would be arbitrary.
There are some people here from much later than that, a few from Medieval times, some from the early 1900s, and some who aren't from Earth at all. Since most arrivals are from the early 21st century it made the most sense just to use that system, and time has marched on. The calendars in the stores have agreed with us