Some bug with the game, the process still running probably, try restarting steam client.
[Edit] Opps i somehow skipped the part with the "pc was off".
Yeah maybe try start and stop the game like OnkeIM said.

Have you tried turning it off and on again?

did both you guys ^^ miss the part where he said the pc was off while the counter kept counting?

i think, steam does not tick every minute a process is running, but has 2 events like, game started and game ended. have you tryed to start the game again and exiting it (through ingame menu)?

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Guys, I followed your advice but nothing happened.

The CRAZY thing is: I decided to remove the game from my library through support, but the counter IS STILL GOING UP. WITHOUT OWNING THE GAME ANYMORE. I'm going mad.

I wrote the support about this, it doesn't make sense.

I think it has to be fixed within the game. I had something similar with Clicker Heroes last year, it crashed and after that the game was shown as being played - only I couldn't play it anymore (got an "the game is already running" error), nor could I delete it ("close the game before deleting" - er, it's closed already XD). I re-installed Steam, and had to reinstall other games I previously had on that PC, but it didn't help. Eventually the game got an update and everything went back to normal.

I bet you never expected the Steamish Inquisition...

Bump for solved, it was kinda hard

It's a known steam/valve bug, it happens often when you use Idle farmers (Idlemaster, ASF etc) and rarely when a game crashes.

As a workaround and a safety measure too, i would go here: (https://store.steampowered.com/twofactor/manage) and click the button "Deauthorize all other devices".

This would force all other browsers and secondary clients to log off. Note that afterwards you will be asked for a Steam Guard code from your phone/tablet.

Look in task manager to see if there is anything relating to the game open. If there is, end it. That's been my problem in the past, but since the time still goes up when the PC is off, it's probably a completely different problem.

Steam bug, although it is usually resolved at the next time you launch the client. Or once you launch the game again and exit it. If you farmed it with IdleMaster or ASF, then it just got stuck and needs the same method to stop "playing".
or do what judgegr said, de-authorise all devices to close all sessions forcibly.

Poltergeist