The Jewish calendar is based on three astronomical phenomena: the rotation of the
Earth about its axis (a day); the revolution of the moon about the Earth (a month); and the revolution of the Earth about the sun (a year). These three phenomena are independent of each other, so there is no direct correlation between them. On average, the moon revolves around the Earth in about 29½ days. The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.
The Jewish calendar, however, coordinates all three of these astronomical phenomena. Months are either 29 or 30 days, corresponding to the 29½-day lunar cycle. Years are
either 12 or 13 months, corresponding to the 12.4 month solar cycle.
In Genesis 1:14 God declared that these astronomical bodies which cannot be controlled or
manipulated by man were the authority in declaring when the Moeds or festivals were to be held. Think of them in today’s terms as the default position when there is no human authority. God gave authority to Israel to take part in determining these Feasts while there was a Jewish court to do it.
In the fourth century, Hillel II established a fixed calendar based on mathematical and
astronomical calculations. This calendar, still in use, standardized the length of months and the addition of months over the course of a 19year cycle, so that the lunar calendar realigns with the solar years. Adar II is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle.
Hillel II, held the office of Nasi of the ancient Jewish Sanhedrin between 320 and 385
CE. Therefore he had the authority to go back to the default position when the Sanhedrin was threatened with extinction if they continued to sight the new moon.
Here at El Shaddai we adhere to this calendar because no individual has the authority to determine the calendar based on their own personal sighting or their own personal barley
crop to determine the beginning of the new year. You can’t have a dozen different people, especially who have no authority, or are not even Jewish, all saying you have to follow their sighting of the new moon or their barley crop to determine the Jewish Calendar. If I put on a police uniform and carry the laws of my state around, I still cannot go arrest people as I have no authority. Well no one else has the authority to change the Jewish calendar.
Just because someone carries a Bible and wears a prayer shawl does not make them an authority.