Yes, perldoc Date::DateCalcLib did give me author information (so, I knew it wasn't an in-house module) and I did check Steffen's modules on CPAN. I even took a peek at Date::Calc to see if it included the missing Date::DateCalc and Date::DateCalcLib modules (it doesn't). If I didn't get an answer here, my next step was to contact the author. (I started here because of the anticipated turn around time; I needed an answer quickly.)

What I was completely oblivious to was backpan. I had no idea this resource existed (and I wouldn't have known to seek it out). Thanks to Corion's response with links, I was able to pull the module from backpan and get it installed successfully. I'll make some effort to migrate to Date::Calc but it won't happen immediately. I found out after posting this question that the module is being more widely used than I realized.