Explain how real-world events can be used as timestamps.
In 2017, a glass jar coated in cement was discovered when repairing a quay wall in Lausanne. It contained a note signed by "Cement Workers of Switzerland" and a copy of one issue of Le Réveil, an anarchist newspaper published on July 7th 1900.
Such time capsules are found once in a while and very often they happen to contain newspapers. While the builders of the capsule probably wanted to give the recepient the idea what was happening at the time, the newspapers act as a way to date the time capsule.
Similarly, when Satoshi Nakamoto developed Bitcoin he has put following text to the first block:
"The Times 03/Jan/2009 Chancellor on brink of second bailout for banks"
It's a reference to the headline in The Times newspaper published on January 3rd 2009. It's a proof that first bitcoing block was created after that date. If it was created before that day quoting the headline would not be possible.
Let's ignore whether such proof has any practical value and let's ask a different question: We know how to prove that certain block was created AFTER certain point in time. But can we prove that it was created BEFORE a point in time?
... explanation of why it is not possible goes here ...
Solution: The Times have published an article called "Bitcoin: Where Did That Come From?" on Dec 15th 2013. That proves that the first Bitcoin block must have been created before that day.