Board Thread:General Discussion/@comment-28211557-20160310122940/@comment-86484-20161211164847

The numbers on their nicknames aren't necessarily related to their birth years, but it makes sense for the game to take place in - or shortly after - 211X because, when Sans first talked to Toriel, she laughed like she hadn't heard such a good joke in 100 years. Now, that, out of context, would still be extremely vague, but Sans has an uncanny ability to figure out precise details from other people's reactions, so he may have known exactly how long Toriel had been in the ruins from her laughter alone. Now, that happened before the 7th human died - because Sans and Toriel talked to each other a few times before she started crying and asked him to promise to protect any humans who came out of the ruins - and Toriel said it had been a long time since a human fell into the Underground, so I'd say it was at least a few years after 211X.