"Oh, I thought it was how long before my *phone* died."

In “Countdown,” the mildly stupid teen horror flick about a phone app that tells you how long you have to live, it’s not the app that kills you. The app doesn’t have any control over your lifespan, either, which is to say, the act of asking has no effect on the answer. The app is just conveying information. Don’t blame the messenger if the time you have left is too short for your liking — as is the case, coincidentally, with most of the young characters in the movie.

Now, if you take measures to prevent your imminent demise and cheat Death, that’s when the app comes into play. Not accepting your fate is a violation of the user agreement. You’ll still die as scheduled, even if you don’t accept the ride from a drunk driver or take the plane trip that was supposed to do it, only now you also get harassed by a demon beforehand. But, again, you still die. It’s like “Final Destination” without the cleverness, “The Ring” without the scariness.

(A better version, aimed at people over 40: The app tells you when you’re going to die, and the horror comes from finding out you still have like 40 years left.)

Our plucky heroine, Quinn (Elizabeth Lail), is a newly credentialed nurse who works for a handsy doctor (Peter Facinelli) at a hospital that conveniently has an abandoned wing perfect for spooky third-act battles with the supernatural. Quinn is displeased when she downloads the app and learns she has only three days to live, and replacing the phone doesn’t help — the app shows up again anyway. At the phone store, run by a needlessly rude hacker (Tom Segura) who’s good for a few chuckles, Quinn meets a young man named Matt (Jordan Calloway) who’s in the same boat. The two seek the aid of an excitable Catholic priest (P.J. Byrne), possibly a stoner, who’s found snacking on communion wafers and is REALLY into demons and stuff. He, too, is slightly amusing.

The work of first-time writer-director Justin Dec, “Countdown” is the kind of painfully generic horror throwaway that usually comes out in the movie graveyard of January, not plum spots like Halloween. It follows the template with precision (pre-credits scene of a random death, guilt-ridden backstories for the lead characters, ghostly sights in mirrors that vanish when you turn around to look), without enthusiasm or variation. The basic problem with the premise — just don’t download the app, dummy, and then you won’t be tempted to try to change your fate — is not addressed. It’s while wasting 90 minutes on mediocrities like this that you start to appreciate how short life really is.

Crooked Marquee

C (1 hr., 30 min.; PG-13, brief strong violence, a little profanity.)