The code in core_ext/string/conversions.rb:45 will result in throwing "NoMethodError - undefined method `<' for nil:NilClass:" if given an invalid date. Should it return nil or maybe throw something like ArgumentError?
This is actually an error in ruby, not rails.
It can, however, be fixed in rails. And has the advantage of fixing it in every version of ruby.
I've fixed it in #3097, waiting to be merged.