Clarification questions about timestamp servers
In 1991, Stuart Haber and Scott Stornetta proposed a method for time stamping a digital document. In their method, when someone (Alice) wants to timestamp a document, she send the hash of her document to a Trusted Third Party (TTP). TTP (similar to figure 1) puts this hash in a Merkle tree and sends back to Alice a proof that her document is in the Merkle tree and then publishes this Merkle root [RH(i-1)] in a newspaper or Usenet post, just for time stamping, adding a specific time (publishing day) to RH(i-1).

Next day, TTP does a similar job with a difference: for calculating the Merkle root RH(i), he uses the previous day's Merkle root [RH(i-1)]. It is called linking and hash pointer and linked list made it possible. Hash pointer (figure 2) is a pointer to hash of some information that it will guarantee that the information hasn’t changed, because if someone wants to change a previous hash, he should change all next hashes and informations in the chain.

Bitcoin uses a hash pointer (figure 3) for creating the ledger of transactions (Tx), with a difference: for calculating hashes of the chain, it uses Hashcash. Hashcash added the proof of works to bitcoin. In hashcash, miners scanning for a value that when hashed, the hash begins with a specific number of zero bits. For finding this specific hash, miners changes Nonce parameter in the blocks of chain.

In bitcoin, with a peer to peer network, hashcash-based timestamp server and the ledger of transactions is distributed and all miners have a copy of it, so bitcoin doesn't need a Trusted Third Party (TTP).