Board Thread:Questions and Answers/@comment-96.23.142.157-20140601175617/@comment-24670104-20141227192354

Furthermore, if the Mountain Men do not possess a solarium to calculate the hours based on the sun, they could have thought of a new timeframe early on in the 97 years inside Mount Weather and their 2141 could be in the 2150s or later.

Also, on the Mir-3 page, it says that the current year for the Sky People is between 2150 and 2199 so it must be sometime between those two years for them but 2141 for the Mountain Men and maybe even a different year for the Grounders, if so it would probably be 97 now, as that is how many years since the nuclear war.