I would not be surprised if the state of unit tests on good date parsing libraries are not sufficient to design a new one from scratch.

See the number of unit tests in the Linux kernel, for example.

You might be right, I haven't checked. It just seems on the face of it such an easy thing to test. Scalars go in, scalars come out. (This could just be me doing the Dunning-Kruger thing).

You could run a fuzzer against two libraries at the same time to find discrepancies....... hmm. That might actually be a good exercise.