“Time Bomb Y2K,” an HBO documentary about the panic related to the calendar switch to 2000 and the pandemonium “experts” warned would ensue, possesses an “Everything old is new again” quality, a semi-comical reminder that conspiracies and media hysteria don’t end but merely recycle. The only negative aspect is, by relying entirely on archival material, those featured needn’t answer today for embarrassing things they said and did then.