This is pretty cool! I really like that dates (and timezone info) are first class types. Often neglected, time ser/de has been a monkeywrench in many a project.

Protobufs are neat but `protoc` is an absolute nightmare. I would warn against going down the path of transpiling to lang-specific message files if at all possible (I don't think you are taking that approach, but github freezes my phone browser, I'll look more on my dev machine)

Great stuff! I would love to see this get more traction. Interchange formats are a classic victim of critical mass necessity.