Thank you for your interest. Every calendar has idiosyncrasies, and God doesn’t reveal many details about the subject in His Word.
Which calendar should the Church use was a question that the late Herbert W. Armstrong addressed in the 1940s. There was division with the Church of God (Seventh Day) at the time, which led to questions within the congregations Mr. Armstrong pastored. In the spring of that year, he wrote to the membership, announcing that extensive study (with the assistance of some of the members in the Eugene , Oregon , congregation) led him to conclude that the Church of God should use the Hebrew calendar to determine the Holy Days.
Controversy over the same issues that the Church addressed in the 1940s resurfaced in recent years. In the spring of 1996, the United Church of God prepared a preliminary study on the subject and invited those who wished to submit papers for consideration. After approving a doctrinal study paper on the subject, the Council of Elders mailed a copy to the entire UCG ministry in February 1997. Here is the conclusion:
“After a study of all the issues and papers, the Council of Elders has come to the conclusion that the Hebrew calendar is a valid calendar for Christians to use in setting the dates for the observance of God’s Holy Days. While major issues seem to revolve around the four rules of postponement ( Dehio th ), the conclusion of the Council is to accept the calendar using the current calculations and rules. The rules of postponement are used in the establishment of the first day of Tishri. Once the first day of Tishri is established, then the calendar can be set, and the first day of Abib can be determined. This, in turn, makes it possible to establish the dates for the Holy Days. It is incorrect to refer to the rules of postponement as ‘moving’ the Holy Days. While we must always be willing to listen and learn, in the absence of any new information, the Council has chosen to confirm its support for the use of the Hebrew calendar as the official position of the United Church of God, an International Association.”
The following quotation illustrates just how accurate it is, as well as the need to make slight adjustments occasionally:
“The calculation of the calendar was transmitted to the sages in an unbroken chain going back to Moses … According to the ancient calculations, the exact time between one new moon and the next is 29 days, 12 hours, and 793 chalekim ‘parts of an hour’ (the hour is divided into 1080 parts). In other words, one lunar month has 29.53059 days. It is interesting to note that according to NASA (National Aeronautics and Space Administration), the time between one new moon and the next is 29.530588 days. Of course, NASA has at its disposal the most advanced and sophisticated telescopes and computers. Nevertheless, the difference between NASA’s figures and that used by Hillel II, which originated more than 3000 years ago, is .000002 or two millionths of a day, calculated for the period one month” ( The Essence of the Holy Days—Insight From the Jewish Sages by Avraham Yaakov Finkel, 1993, p. 141).