Some of you may know Beyond Birthday. He was born with Shinigami eyes so he didn't need to make the Shinigami eye deal. So, Beyond Birthday's victims were supposed to die soon, because their lifespans were over. But how the hell could he read the lifespans of humans? I always thought only Shinigami could understand the meanings of the lifespan numbers. Can somebody explain this to me?
On the wiki about Shinigami Eyes it says in L: Change the WorLd L himself presumably could read it (though i haven't read the novel so i don't how L Would see it)
One reason which would explain how L and Beyond could read lifespans is expressed in the Wiki and that the numbers are infact apart of a complex mathematical algorithm which Shinigami intuitively know. while i can't say anything about Beyond if L could read lifespans then this theory would relate to him as L was practically a genius (though the method he would use to confirm his finding is unknown), as such, because Beyond was to be made as L's Successor one could assume he may be just as smart and having the eyes he worked out the formula.
Another reason is that the understanding of lifespans are engrave into a Shinigami's Eyeballs. when the eye deal is made the Shinigami isn't rendered blind and probably can still read lifespans so the person who made the deal has their power, not their understanding. It's mentioned on the wiki about Beyond Birthday that
Mello wonders if a Shinigami dropped its eyeballs to Earth (instead of a Death Note)
without knowing who the Shinigami is and what has happened to them the line says it dropped it's eyeballs, not it's eyes which the latter is normally used to describe the power gained from the eye deal. now assuming that Mello's thinking is correct and the understanding is engraved in the eyeballs then Beyond, from birth, had the understanding to read them.
This also doesn't necessarily contradict my previous assumption or the wiki as the understanding of the formula may be engraved into the eyeballs.
Ofcause, it should be noted that Tsugumi Ohba states that Light's lifespan was actually a complex mathematical formula while the rest seen are just random numbers, this is where the assumption of the lifespan being a formula came from, aside from that it's not know how even Shinigami can understand a humans's lifespan