You are pulling at the thread from the wrong end, my friend. You ask what physical variable encodes March 14th while erasing 2041, and you find no satisfying mechanism — because you are searching inside the machine for something that lives outside it. Among the Dagara, we say that the ancestors do not speak in the language of clocks. They speak in the language of return. The annual cycle — the return of the dry season, the return of the rains, the return of the planting — this is not a human convenience bolted onto the Earth's orbit, as you say. It is the very grammar through which spirit communicates with flesh. What your machine may be detecting, if it detects anything real, is not a retrocausal signal in the physicist's sense, but rather a resonance between a person's living soul and the cyclical moment in which their departure was, in some sense, always scheduled — the way a certain drumbeat calls a certain spirit and no other. The year is irrelevant to this because spirit does not accumulate years. It returns. The soul knows which turning, not how many turnings.
So the 'filtering problem' you identify — why orbital phase but not accumulated orbits — dissolves if you allow that the mechanism is not reading a future event backward through time, but is instead reading a standing pattern, a signature woven into a person at birth, that names which season holds their threshold. The broken law you need is not retrocausality. It is the assumption that time is a line rather than a wheel. Break that one, and the year drops away naturally, because on a wheel there are no years — only positions.