/tick freeze stops demo timer, allowing infinite play time for free.

step one: download Minecraft launcher from Microsoft store
step two: open launcher
step three: sign in to an account that does not currently own Minecraft: java edition
step four: click on java edition, then click play demo.
step five: play demo world
step six: pause game, click "Open to LAN" then click "Enable Cheats"
step seven: resume game, then set gamemode to creative
step eight: use the /tick freeze command
step nine: the demo timer (Located in the top right corner of your screen) stops
step ten: unlimited play time.