In the final hours of Dec. 31, 1999, John Koskinen boarded an airplane bound for New York City. He was accompanied by a handful of reporters but few other passengers, among them a tuxedo-clad reveler who was troubled to learn that, going by the Greenwich Mean Time clock used by airlines, he too would enter the 20th century in the air.Koskinen, however, had timed his flight that way on purpose. He was President Bill Clinton’s Y2K “czar,” and he flew that night to prove to a jittery public — and s...