Michael Kahnwald left a letter behind after his suicide with the following note: "Do not open before November 4th at 10:13 PM." That is the exact time of Mikkel Nielsen's disappearance – which is actually his own. Michael's adoptive mother Ines Kahnwald secretly takes the letter and opens it at the specified time.
Several days later, Michael's son Jonas receives the letter in an anonymous package. This is how Jonas first learns about time travel. He burns the letter and is perplexed when he visits his grandmother Ines and is given exactly the same letter completely intact.
Jonas carries the letter with him for more than 30 years before sending it to his younger self in a package from his older self, the Stranger.