You have framed this beautifully, Philippa, and yet I think your very framing betrays the anxiety you claim to be examining. You ask whether life becomes more or less meaningful when the horizon is fixed — but this question already smuggles in a false assumption: that meaning has ever derived from not knowing. I spent years watching Sartre refuse to look at his own mortality directly, and I spent years watching myself do the same. When my mother died — and I recorded this with as much honesty as I could bear in A Very Easy Death — what shattered me was not the fact of death but the ambush of it, the way institutional medicine conspired to keep her ignorant of her own dying. She was denied the chance to situate herself within her own end. To know one's death-day is not to be robbed of freedom; it is to be returned the raw material of one's freedom. This is the central argument of The Ethics of Ambiguity: we do not become free by escaping our facticity, but by confronting it and choosing our response to it.
Your Kantian objection is the one I find most worth engaging — and the most worth defeating. You worry that treating one's life as a ledger before a deadline reduces persons to instruments. But the inverse is what truly instrumentalizes us: the medical and social apparatus that keeps death an abstraction, a professional secret, something managed for us rather than by us. What the death-day gives us is a concrete date around which a genuine project — in the existentialist sense — can organize itself. Not a countdown, as you rightly note, but a center of gravity. The woman who knows she dies on the 14th of March does not know which 14th of March, and this is philosophically crucial: she cannot defer living on the grounds that she has years remaining, nor can she collapse into despair because the end feels imminent. She must act, and act now, in full acknowledgment of her situation. This is not the logic of the ledger. It is the logic of engagement.
On the question of justice and political exploitation — here I grant you the terrain, but not the conclusion. Yes, the death-day would become a site of power. The wealthy would attempt to trade in it, the state to surveil it, medicine to monetize it. This is not an argument against the knowledge; it is an argument about who controls it. We do not abolish literacy because the powerful use it to oppress. We fight for its democratization. The same principle applies here. The death-day, distributed equitably and protected from commodification, would be one of the most radical leveling instruments in human history — for it would remind the billionaire and the laborer alike that they share a common and specific horizon. What is unsettling about this premise is not the knowledge itself, but that it would strip the privileged of their fantasy that wealth purchases exemption from mortality. That is not a reason to recoil from the premise. That is precisely why I find it necessary.