I'm not making judgements on anybody for reposting it. I did, but my reason was out of sheer glee at seeing the responses. Some posted it in an attempt to analyze why it creeped them out, some posted it out of a vague superstitious feeling, some posted it as a lark.
The whole thing resonates on the same level as the Japanese Horror ghost genre, made popular here in the western world by movies like The Ring (Ringu) and The Grudge. I have a particular soft spot in my heart for Japanese ghosts, because they behave differently than euro-american ghosts, and are more... physical, more material, yet they have a connection to the same etherial/otherworldly nature of ghosties and beasties that go bump in the night.
I like it because the concept opens up these huge vistas for storytelling. The "ghost" is created by the negative energy that remains when someone dies through violence, or anger, or without being able to accept their smooth transition into the next world. That negatrive energy lives on, and affects those who come into contact with it regardless of their culpability. That's the important distinction between the Asian and European ghosts: the western ghosts tend to take revenge upon the perpetrators based on some sense of moral outrage, where the eastern ones affect anybody. It's an unsettling difference, because it creates the illusion that you are vulnerable, even if you're not involved, and the psychological impact of that can go very deep indeed.
Another aspect of the "900 seconds" thing is the girl without a face. This is another J-horror element that I've seen the most in the Silent Hill game series, where it is a truly effective element-- the freaky nurses are scary as hell.
As human beings, we resolve to faces from birth, and center on the expressions of faces as being human and expressing emotion. As an experiment, if you draw two dots above a line on a piece of paper, almost everyone will recognize it as a face, and depending on the shape and relationship of the elements, will garner some sort of emotion being expressed. It's a version of anthropomorphism, where we attribute human characteristics to non-human objects: we see faces in things where there really aren't faces.
The faceless girl is sort of a reverse-anthropomorphism, where taking away the face from something that is recognizably human removes that avenue of connection, and renders that moving thing as something disconnected from humanity. It triggers an ancient fear of being stalked, of being hunted. That's an instinctive fear, and it's not one that is easily conditioned away.
Slight aside here: in the Silent Hill games, there are these creatures that look like you took the bottom half of two mannequins and stuck them together at the waist, so one pair of legs walk, while the other pair are free to kick you. It's another creepy visual element that has the same sort of reverse anthropomorphism that is so delicious.
The use of 900 seconds rather than 15 minutes is interesting. 900 seconds is a more definitive deadline, it's a sharper image than 15 minutes, probably because we are conditioned to do things in 15 minute blocks, and they have a kind of familiar granularity. 900 seconds is more exotic, less familiar, giving it a sharper deadline feel. By itself, that's not particularly scary, but in the context of the already-creepy, it adds to the feeling like a spice adds to cooking.
Oddly, I don't think this was a conscious choice by whoever came up with it. I think they just tapped into things they thought were creepy mostly by accident, but cashed in on a wealth of psychological imagery.
And it truly gives me ideas.