Parshat וילך began with Moshe Rabbeinu telling the people that he was 120 years old that day. He knew that this was going to be the last day of his life, despite the fact that he had not lost any of his strength.
What was implied by Moshe’s words was that he would pass away on his birthday. Although the Torah only gave the actual date of death by Aharon, (first of Av) we have enough information from the text to let us know Moshe’s date of leaving this world.
We know that there was a mourning period of thirty days following Moshe’s passing. We are also told that the Jewish people entered Israel on the tenth of Nissan, three days after the thirty day mourning period for Moshe. This means that Moshe died thirty three days before the tenth of Nissan, clarifying that his date of birth and death, was the seventh of Adar.