Island in the Sea of Time

It's spring on Nantucket and everything is perfectly normal, until a sudden storm blankets the entire island. When the weather clears, the island's inhabitants find that they are no longer in the late twentieth century...but have been transported instead to the Bronze Age Now they must learn to survive with suspicious, warlike peoples they can barely understand and deal with impending disaster, in the shape of a would-be conqueror from their own time.