In the beginning a perfect system of 12 thirty day months making a 360 day year was established and all nations used a 360 day year calendar. The new moon began after the end of every thirty days.
During the reign of Hezekiah the cycle was disturbed making careful observation necessary to establish the new moon and the beginning of the months. See the Biblical Calendar articles for more.
We are told that when Christ comes there will be massive earthquakes all over the earth so that the entire surface of the earth will be remade. I suggest as speculation that Christ will restore the correct intended lunar solar cycles at that time which may be a big factor in causing those earthquakes.
Personally I think that Christ will restore the proper cycles, but we live in the now and we need to learn and exercise wisdom.
Over the years I have come to the following conclusions.
1) God set the sun and moon in motion and commanded that the light be used to establish days, months and years (Gen 1:14). Throughout history until the modern Rabbins decided to change God’s Calendar away from the first visible light to the darkness of molads: The months for Israel ALWAYS began with the first visible light of the new moon. Again See the Biblical Calendar articles for the history and further details.
2) If the light of the new moon is calculated to be visible, and it is observed; then the new month should be consecrated.
3) If the new moon is calculated to be visible and it is NOT seen at Jerusalem due to a lack of witnesses in that city, cloud cover or other adverse conditions; but it is seen in the land or within a short distance east of the land by at least two witnesses; it is evident that the moon could have been seen at Jerusalem based on the two local observation witnesses and calculation and the new month should be consecrated.
4) If the moon is calculated to be visible at the end of 29 days and it is NOT seen by witnesses [subject to statement #3] then it should NOT be consecrated, and should be delayed to the next day.
5) If the moon is calculated to be visible at the end of 30 days it should be consecrated regardless of whether it is seen or not; since the moon cycle is 29 1/2 days and no month may be longer than 30 days.
6) Since binoculars and telescopes are a modern invention, and since locating the moon is an integral part of sighting the moon; The first visible light of the new moon must be both located and seen by unaided normal eyesight [corrective lenses to 20/20 are permitted].
The New Year should be consecrated when enough barley in Jerusalem will be ready for the Wave Offering by the 14th day of the first month.
In rare years where there is no ripening barley do to great drought, the new moon CLOSEST to the spring equinox should begin the year, so as to prevent a year from being spread out over more than one complete proper year do to a lack of harvest.
To use the new moon BEFORE the equinox as a hard rule, could [in some years] start the month well before the harvest would be ready. To use the new moon AFTER the equinox as a hard rule could in some years result in the Festivals being delayed until they are out of season, for example putting the Fall Festivals in November.
The Weekly Sabbath begins when the sun sets locally. Persons living north or south of 55 degrees may observe the Sabbath from 18:00 hours [this is the average sunset time at Jerusalem], and all of the brethren in any such location should observe the Sabbath at the same time by common accord. When Christ comes he will decide on this matter.
Certain corporate COG people have ridiculed the Biblical Calendar saying that there are numerous calendars out there.
That is true, however there is ONLY one Biblical Calendar which is true to Holy Scripture and true to the Calendar used during the history of Israel through the ministry of Jesus Christ and beyond: and the modern Rabbinic Calendar which they use is NOT it!
Today’s modern apostate from Scripture Rabbinic Calendar was not finalized until 1178 A.D.
This whole issue is mainly a test of our zeal to please God.