Rabbi Yoshiyah raised a question that touches the very structure of the Jewish calendar: who has the authority to add an extra month to the year? The Hebrew calendar is lunar, and periodically an additional month — a "leap month" — must be inserted to keep the festivals aligned with their proper seasons. But this is not a decision any local court can make.
Rabbi Yoshiyah taught that the year may only be intercalated by the Great Beth Din in Jerusalem — the supreme court, equivalent to the Great Sanhedrin (the supreme rabbinic court). His proof comes from the Torah's own language. When God declares "This month shall be the first for you," the command is addressed to "Moses and Aaron" — the leaders of the entire generation.
The emphasis falls on the word "you." The Torah does not say "the first month" in the abstract. It says "the first for you" — directing the command to the highest authorities of the nation. Moses and Aaron were not local judges. They were the leaders of all Israel, analogous to the Great Sanhedrin that would sit in Jerusalem in later generations.
From this, Rabbi Yoshiyah derives that only a court of equivalent stature — the supreme judicial body in Jerusalem — holds the power to intercalate the calendar. The determination of when months begin and when leap years occur is not a local matter. It belongs to the central authority of the nation, because the calendar governs the timing of every festival, every sacrifice, and every sacred observance for all of Israel.