Bitcoin Whitepaper Song 08-1 - Timestamp Server 

The solution we propose
begins with a timestamp server.
A timestamp server works
by taking a hash of a block of items
to be timestamped
and widely publishing the hash,
such as in a newspaper
or Usenet post.
The timestamp proves
that the data
must have existed at the time,
obviously,
in order to get into the hash.
Each timestamp includes
the previous timestamp
in its hash,
forming a chain,
with each additional timestamp
reinforcing the ones before it.