我觉得你想说的是“并行的历法”，right？ In China, we also use two kinds of calendar that could be found, one is Calendar Year or Gregorian calendar; one is Lunar Calendar, this is what you are talking to , I think. But how about your country?

Okay, I see. I would say '平行世界'(Parallel World), occasionally '平行宇宙'(Parallel Universe). It often happens in a sci-fi (novel).

并行存在

An alternative 2015 so to speak.

Hi, Hunter. my meaning is the story is set on earth, in our time, but the world is different to what we know. In the story humans have achieved different things to what we actually have.