Harold Camping seems to be repeating the mistakes made by the early Adventist movement. While I disagree profoundly with Seventh-day Adventist eschatology, I don’t hold that history against the church. Good and interesting movements can result from mistakes, but only if you correct those mistakes.
Now consider Camping. He predicted the rapture in 1994, and then decided his math was wrong. Those who know Adventist history will likely recall the 1843 “lesser disappointment.” After Jesus did not return in 1843, William Miller corrected the date and also made it more specific, narrowing it to a single day, October 22, 1844. That day is known in Adventist history as the Great Disappointment.
Following that event, Adventists decided that, while they had the math right, what had happened was a change in heaven, as Jesus began the investigative judgment, which is still going on now.
They also, however, acknowledged that they were wrong to try to set dates for the second coming. On this last note, Camping is not following in their footnotes. He appears set to repeat their mistakes, but not follow their example when they did right.