A dude named Alec Schaal posted a series of videos this week that has the internet equally mesmerized and stumped -- because it shows what appears to be a dude sneaking onto his property, going into a shed ... and coming out a day later looking way different.

By that, we mean the man purported to be a "time traveler" emerges several years older.

Take a look and you'll see what we mean. Alec says he's got a home in Florida that he rents out as an Airbnb -- and there's a security camera system that alerts him to activity when he's not there. He got pinged to some movement, and says he saw something incredible.

Check it out ... the video appears to show a random young dude wandering onto Alec's property off the street and he moseys around aimlessly before popping into a shed, seemingly for no reason. Alec calls the cops, who come to check it out -- but can't find the guy in there ... even though you never actually see him leave the space. A day later, a big update.

Alec posted footage that seems to show a totally different person exiting the shed out of nowhere -- only this man is elderly ... although, he looks awfully familiar, at least to Alec.

Long story short ... Alec thinks he's got a time traveler on his hands -- and he insists there's no trickery or pranks afoot on his end. Some believe it, others are a bit more incredulous.

Alec says there are no tunnels in his shed, and he's going to the house soon to inspect what could possibly explain this. On its face, however ... you could call it 'Back to the Future,'-ish.