I also do not know how Camping came up with his 6pm timestamp, apart from knowing that he was wrong.
As for the Joplin tornado, it did not happen at 6pm. Touch down was at 5.41pm. Source: The tornado touched down at 5:41 p.m. Sunday -- 24 minutes after a warning was issued -- said Thomas Schwein, central regional director for the National Weather Service. It grew to as wide as three-quarters of a mile at one point along its estimated four-mile track, according to Missouri's State Emergency Management Agency.
As for the volcanic eruption in Iceland, Grimsvotn erupted at 5.30pm local time on Saturday. Source: Satellite Photos Show Grimvotn Volcano Eruption From Space - 2 hours ago The Grimvotn volcano began erupting Saturday at 5:30 pm local time on May 21. ... [Photos of Grimsvotn Volcano Lightning Show]
Thus, none of the two events happened at 6 pm local time, unless rounding up is allowed (and if that is the case, then it is a guesstimate rather than a prophecy). Bottom line is that Camping is a false prophet no matter how the bread is sliced, and that is based on Biblical measures for a false prophet (if one prophesies something and it does not happen). His day was off, and his timing was also off.
Unless ofcourse there is rounding off in heaven.