English: It is written that Aaron died in the fortieth year in the fifth month (Av), and later that Moses spoke in the fortieth year in the eleventh month (Shevat). Since both Av and the following Shevat are called “the fortieth year,” the New Year for counting from the Exodus is not Tishrei — years of the Exodus are counted from Nisan.
Type of Learning
Inference from parallel dating (same “fortieth year” spanning Av and Shevat) → calendar of the Exodus begins in Nisan.
What Is Learned
The dating of Aaron’s death proves that the years from the Exodus are reckoned from Nisan, not from Tishrei.