The Biblical Calendar is known as the original calendar of the Scriptures. It seeks to follow a calendar based on a timeline established in the Old Testament. The rationale for following the Biblical calendar lies in the belief that the only way to observe feast days on the correct days is to follow a calendar which is true to the days intended by the Bible. In the Jewish faith, this is particularly important, as this passage from Num. 9:13 illustrates:
"But the man that is clean, and is not in a journey, and forbears to keep the Passover, even the same soul shall be cut off from among his people: because he brought not the offering of Yahweh in his appointed time, that man shall bear his sin."
In other words, it is literally a sin to not observe feast days at the proper time.
In the Old Testament, instruction for observances of feast days was based on the day and the month, although no specific month was named since this concept had not even come into existence. Instead, the instructions were along the lines of “on the tenth day of the first month.”
For the ancients, a day was defined as starting at sundown and ending at the next sundown. This is known as Byzantine time. It may seem odd at first, but it makes sense: most people are generally awake at sundown, while most are still asleep at dawn. Also, it is easier to determine when the sun sets than when it rises, as you see it progressively getting lower in the sky. Although starting a day at sundown may seem odd at first, it is really no more odd than our system of having a day start at midnight.
In the Biblical Calendar, weeks are determined by the lunar cycle. Months, too, are based on lunar observances. Throughout the Bible are references to specific times at which feast days are to be observed, based on the moon’s cycle. And, as proponents of the Biblical calendar argue, it plainly states in Psalm 104:19: “He appointed the moon for seasons.”
For an overview of how Biblical time works, here is a breakdown of days, weeks, months and years, along with the biblical passages where these are referenced:
For many years, there has been extensive debate over whether or not the Hebrew calendar (aka the Jewish Calculated calendar/Hebrew Calculated calendar) can be considered the true Biblical calendar or not. In 2007, the controversy hit home when Don Roth was challenged by a group contending that the sighting of the first visible crescent of a new moon is the correct way to determine the Sabbath
Don Roth, an engineer by profession, took it upon himself to look into the matter. Using his engineering background, he conducted a mathematical analysis of the Biblical calendar and its relationship to the Hebrew calculated calendar. In his free 3-DVD collection, Don explains his mathematical proof for why the Hebrew calculated calendar is true to the Biblical calendar and can be considered the true calendar of God.