The Nantucket series is one of two timelines established by S. M. Stirling in The Change universe, the other being the Emberverse. It is the preceding and originating works of the universe. The quirk of Emberverse's start in 1998 is established as the time that the first novel of the Nantucket series was written.
The Nantucket-verse differs from the Emberverse, in that science and technology works as normal, it is just a temporal transposition to the residents of Nantucket Island. The island is timeshifted several thousand years into the past, and is replaced in 1998 with its equivalent from several thousand years ago.
Evidence of this temporal transposition is given when the Quest Fellowship of Emberverse arrive on Nantucket Island and encounter Paleo-Amerinds of that time, and virgin old-growth forest.
Several characters from the Nantucket-series have relatives in the Emberverse.
The 1998 residents of Nantucket believe that the people left behind were left in a better place, as they did not have to survive the savage uncivilized world of the ancient past, not knowing that all technology had broken down and that civilization would fall after The Change that transported them to the distant past. This is a view held by Marian Alston as well.