While this might not seem exactly like a programming question, non-programmers won't know what I'm talking about. If you can suggest a more appropriate stack* forum, I'll happily ask there, but I think this is my best shot.
libical is a good start, but it doesn't have anywhere near what I need. I do not want to reinvent tons of calendar math functions if they already exist, and I also do not want to suck in boost or roguewave or anything like that just to do calendaring.
Any suggestions? I've looked and looked and found nothing, but my google-fu isn't supreme.
By rejecting Boost you're rejecting a library that's had not only extensive design review but also probably extensive testing by people who you can guess (hope) have experience with the picky picky details of datetime calculations.
But ok, I can actually understand the desire to avoid Boost if possible.
But you're entering a world where you have even more responsibility than usual to validate the design and implementation.
That said, this library looks like its promising for your purposes: Howard Hinnant's date library on github (see also his pages here for a man page and here for a description of the algorithms used. I have not used it, of course. (If you do take this suggestion, please report back here, so we'll all know about this library's worth.)