The error does not happen in the code you posted. It means somewhere you have something that is supposed to be an NSDate and instead it’s nil.
As for the humor in the error message, I have never seen that message, but +1 to Apple. Some engineer in the Cocoa division is as funny as the good old engineers Apple used to have a lot of time ago.
Check out the kind of diagnostics Apple’s MPW C compiler used to spit out http://www.netfunny.com/rhf/jokes/91q3/cerrors.html