Fair instinct — and you don't have to. On your own machine, put the real secret in a file and password-protect it with a zip tool (or similar) you already trust, nothing web-based. Make up a password for that zip, and seal that here instead of the real one.
This page, and anyone who ever reads its source or its traffic, then only ever sees an arbitrary password you invented for a zip file — never the real secret inside it. When the cipher opens, you get the zip's password back, unlock the zip locally, and the real secret never touched this page at all.
The basic move. Generate a password here, set it on Steam, seal it, log out. Until the unlock date arrives you physically cannot log back in — the decryption key is published by a worldwide network of servers only when that moment comes, so it does not exist anywhere on Earth before then. Changing your clock, going offline or reading this page's source achieves nothing.
Series. Instead of one cipher, mint a run of them — daily, weekly, monthly, yearly — each opening on its own date, all holding the same password. Open one, play, then burn it: you are locked out again until the next date.
Ladder. Gaps that widen over time: every day, then every two, three, up to a fixed weekly slot. Tapering beats cold turkey, and a predictable rhythm beats an erratic one.
Burn after reading. One button deletes the cipher you just opened and wipes the clipboard, so a used cipher can't be reused.
Backups. Email the ciphers to yourself or download them. They are safe in plain sight — a stolen cipher is worthless until its date. Lose every copy, though, and the password is gone for good.
Close the back door. A lock on your password means nothing if you can reset it by email. Hand your account's recovery email to someone you trust, or the whole thing is decorative.