1,240. The Calendar is Only to Be Set in Israel

Hilchos Kiddush HaChodesh 1:7

There is a mitzvah in the Torah for the court to calculate in order to figure out whether or not the moon will be seen, to examine witnesses so that the moon can be sanctified, and to send messengers to inform the people which day was Rosh Chodesh so that they would know the days on which the holidays fall. Leviticus 23:2 says, “so that you will announce as days of holy gathering” and Exodus 13:10 says, “you shall observe this law in its appointed time.”

Hilchos Kiddush HaChodesh 1:8

Making the calculations and establishing the months and the leap years was only done in Israel as per Isaiah 2:3, “Because the law will emerge from Zion (a name for the Temple mount) and the word of God from Jerusalem.” If a scholar who received the original ordination in Israel left for another land without leaving a replacement of his stature behind in Israel, then he could make the calculations, set the months and institute leap years outside of Israel. If he becomes aware that an authority equal to or greater than him has arisen in Israel, he may no longer set the calendar and make leap years from outside Israel. If he attempts to do so when he shouldn’t, his efforts are ineffective.