This is an error message of a runtime exception thrown by NSCalendar:

I mean really, what do you think that operation is supposed to mean with a nil date?
An exception has been avoided for now.
A few of these errors are going to be reported with this complaint, then further violations will simply silently do whatever random thing results from the nil.
Here is the backtrace where this occurred this time (some frames may be missing due to compiler optimizations):

Hurts my newbie objective-c dev ego a bit :–)