Is editing level.dat illegal [closed]

I’m playing the Minecraft demo and I want to give myself more time. I wish to edit level.dat to achieve that goal, but I don’t know if it goes against the EULA.

Illegal according to what?

There is nothing criminal with regards to you editing code that's on your computer in this situation. The EULA is an agreement between you and the company. Breaking it means they can ban your account (but they can ban your account for no reason anyways). There won't be any criminal liabilities unless you sell your modification and/or services to perform this.

In addition, you are fully liable for your modifications. If your code edits somehow causes collateral damage, whatever that may be, you become fully liable for your modification.