UPDATE: platformio is a complete fail. I was getting unusual data from the DS3231 clock, but I couldn't tell if it was my wrapper or the original library that I'd grabbed for the Arduino IDE.
Long story short, I tried renaming the original library folder, installed the first DS3231 library from the platformio gui (after I hunted the gui down) and got the same result.
Ok, no big deal, let's see what the library is doing with the hour/h12/pm data.
Wait, where is the library? It's not in the lib directory. I guess it's global?
So where is the global library folder? It's not visible in the project folder/file list on the left.
I hunt down the library gui button that has the 'reveal in finder' button, and resort to opening the .cpp file in.... TextWrangler. What a waste of time. I can't edit the .cpp file in platformio? I can't select to open it with Atom - platformio just throws a big red error box and pukes.
I'm giving up on platformio for now; it's library system is overly complicated/obtuse/opaque, and I can't easily open my own global library files to edit them at the same as my source code. It is in active development so I can see it getting better, but for now there isn't any difference to just opening everything in TextWrangler and recompiling in the Arduino IDE.