Static members need to be both declared and defined.
For your case, you need to add this in one of the .cpp files (preferably the one that's paired with the header):
1 2
std::string strMonth;
//(Repeat for any other static members.)
Thank you VERY much helios. That worked out perfectly! I've been trying to figure this out for a few days, but I could not get it to work no matter what I did. Once again, thanks!