In order to reset the timer, you’ll have to follow this guide. Basically, you’ll have to delete your local save files, disable Steam Cloud for the demo, and download Steam SAM. After that, your timer will be reset.

1. Disable Steam Cloud for RE8 Demo
2. Download Steam SAM (Steam Achievement Manager)
3. Delete Local Savefiles
(Located: D:\Steamuserdata\\1541780\remote\win64_save
Delete those Files.
4. Start Steam SAM
5. Select RE8 Demo
6. Remove Achievements and Stats, confirm.

If you run the Demo now, Timer is Reset to 1Hour.
You can do that in a couple of Seconds, i even wrote a small tool to do that for me...

Info: If you do not want to set the Settings again all the Time, Reset everything, start the Demo to Main Menu, change all Settings you need and Close the game.
No go to your Local Save and copy data00-1.bin (this is where those settings are stored)
Once you have to reset again, proceed as Step 3 and just add the data00-1.bin you saved to the folder.

Resets the Timer but keeps all settings you made.