1844
"The Great Disappointment" of the Millerite movement. In the preceding years, the self-proclaimed Baptist preacher William Miller had attracted as many as 100,000 followers in New York and Massachusetts. Miller boldly predicted that Christ would return and engulf the world in fire some time between March 21, 1843 and March 21, 1844. When that didn't happen, he changed the date to October 22. Many followers left after that spectacular failure, but enough stayed to form the 7th Day Adventist movement. The type of eschatology espoused by the Millerites is called premillennialism, which holds that the world will grow more and more sinful until Christ returns to usher in the Millennial kingdom--in other words, man can't save himself. Although their failure would serve as a great caution against hard date-setting, historians view them as a harbinger of the type of apocalyptic thinking so prevalent in the 20th century.
http://www.pbs.org/wgbh/pages/frontline/shows/apocalypse/etc/cron2.html
for more information on Miller's chronology, http://www.pbs.org/wgbh/pages/frontline/shows/apocalypse/explanation/amprophesy.html