The trouble with this phenomenon is that it doesn't have a relationship with time. Entanglement is an instantaneous, simultaneous response to stimuli shared between two particles (or two bodies of material, for it's being shown to occur in larger things than first thought) irrespective of distance. And that causes a few headaches with the equations.
The big question is, how does anything get entangled in the first place? Is there a definitive point at which things are entangled, or is it a case, being devoid of any conditional time measurement, that entanglement occurs in advance of its physical evidence? Are these photons (and all other entangled things) destined to be cleaved together and subsequently find each other so paired? Could entanglement, per se, be a real-life physics love story? This is not such a stupid question. For people get entangled just as particles do. Love can lead us a merry dance for a lifetime, and when it does, there's nothing you can do about it.