After hours of attempts at fixes, I've come to discover that the root cause of the issue (including both the initial problem with Howard Hinnant's date.h and the larger problem of errors across ALL of my referenced libraries and headers) came down to the name time.h. Because I unknowingly named a file the same as one that is included in the standard library, and then split it into a separate project and referenced it, all heck broke loose.

Long story short, if you have thousands of random errors across your entire project, just because you moved a few files to a common project and then referenced it, check if one of your headers is named the same as a standard library file.

This problem was WAY too easy to fall into :(

Credit to https://stackoverflow.com/users/4581301/user4581301 for suggesting the problem first. I just didn't think that was it because the application built fine when it was all in one project.