I don't remember for sure, but I think the exact words they used were "alternate universe."
Which means it could be an alternative timeline within the same
universe. Or the same timeline within the same universe:
1. The future, in which the whole world has amnesia.
2. The future, in which all the characters are dead and in heaven (which explains the amnesia).
2a. Except for Locke, who has stayed behind on the Island to be the new Jacob (Locke came back to life; everyone else died.)
3. The past, which has been reset.