EternityWall shows message in the Bitcoin blockchain. Due to economic incentives, the Bitcoin blockchain is the nearest thing to digital eternity.
A message is a transaction with an OP_RETURN output containing valid utf-8 starting with characters "EW".
All dates are referred to the block timestamp containing the transaction and are in UTC.
Languages are automatically detected and they may be wrong.
How to write a message with Bitcoin Core
You can use electrum too, but remember to start with hex `4557` (EW)