FOUR YEARS LATER, few doubt that 9/11 ushered in a new epoch. What's less obvious is that that day also marked the end of an era. For Sept. 11, 2001, was the last day of the 1990s. Of course that's not literally, chronologically true. But just as the great British historian Eric Hobsbawm wrote a history of the 19th century, which, he decided, started in 1789 and ended in 1914, so we can look back on the 1990s and see that they too were not an exact match with the dates on the calendar. That was not obvious at the...