Sealed until later
Generate a password and use it right away — set it as your phone's lock, then hide it. Once hidden, you can't see it again until your timer runs out.
Use it now
The unlock time is checked against real network time, so changing this device's clock won't reveal the password early — but you must be online to unlock. Saved in this browser's local storage on this site: clearing site data, or opening it in another browser/device, loses it. Still a willpower/friction tool — someone with dev-tools access could read local storage before the timer ends.