The world is not permanent, as much as we would like it to be. But we should be wary of people who claim that a particular day (like May 21st) is The Day.
Jesus knew there were people who wanted to know a Definite Day, so he said this:
"Watch out; don't be fooled. Many men, claiming to speak for me, will come and say, I am he! and, The time has come! But don't follow them. Don't be afraid when you hear of wars and revolutions; such things must happen first, but they do not mean that the end is near." (Luke 21:8-9)
The end will be near one day, but it won't be the day a cult leader determines. Only God will do that...
For another good post on this, check out Sheridan Voysey's blog.
Edit: There's a great quote from Eric Landry on Justin Taylor's blog, about how we need to deal with this on Sunday and afterwards...