[usefulcat]

I don't see how this is in any way preferable to having an ordinary default constructor that does the same thing:

    // There are a few ways to let API callers bring their own 
    // memory, as they would in a no-malloc environment and this
    // stack-friendly c'tor is a stand-in for that. 
    static Birthdate epoch() { return Birthdate(1900, 1, 1); }

[plorkyeran]

Some readers will expect Birthdate() to be equivalent to Birthdate(0, 0, 0), and naming it Birthdate::epoch() makes it clear that it is not that. I don't think it's worth it, but there is an upside.