The Biblical Calendar

Both the biblical calendar and today’s Jewish calendar are lunisolar calendars: months are measured by the phases of the moon; days and years are measured by the progress of the sun. There have been months, days, and years ever since Creation, but the biblical calendar didn’t come into being until Exodus 12: “The LORD spoke to Moses and Aaron in the land of Egypt, saying, “This month shall be to you the beginning of months. It shall be the first month of the year to you” (Ex 12:1–2 WMB).

On the biblical Jewish calendar, years begin on the first day of Nisan—the month in which the Exodus occurred. Months begin when the new moon appears.  Weeks consist of seven days.  Days begin at nightfall, following the pattern established on Day One:  “So there was evening and there was morning–one day.”

During Bible times, the everyday Jewish calendar used a new crescent moon to determine when a month began, with an additional month added every so often to correct for the difference between the lunar year and the solar year. The decision to add the additional month was based on observation of natural agriculture-related events such as which crops ripened when. 

The Jewish year found in the Bible is a schematic year consisting of 360 days divided into twelve 30-day months per year. For example, Noah’s flood lasted 150 days (Genesis 7:24 and 8:3). Those 150 days added up to exactly five 30-day months, from the seventeenth of the second month (Genesis 7:11) until the seventeenth day of the seventh month (Genesis 7:24 and 8:4).

The 360-day biblical year of twelve 30-day months has been confirmed by archaeology.  According to Jonathan Ben-Dov of Tel Aviv University,  “Administrators in ancient Judah used schematic 30-day months and a 360-day year alongside other annual frameworks. . . . Material evidence for the 360-day year in Judah comes forth from a series of small perforated bone plaques from various sites in Iron Age Judah. One such item was recently unearthed in the city of David. These objects can reasonably be understood as reflecting a schematic 360-day year, serving as desk calendars for Judahite administrators.”* For example, Solomon’s calendar used a 12-month year: “Solomon had twelve officers over all Israel, who provided food for the king and his household. Each man had to make provision for a month in the year.” (1 Kings 4:7 WEB.)

Today’s Jewish calendar uses a 12-month lunar calendar with an extra month added every so often to keep it in sync with the 12.4-month solar cycle. Because the biblical calendar didn’t work that way, we can’t use today’s Jewish calendar to interpret Bible prophecies. This is true in both Testaments.  For example, Sir Robert Anderson used years consisting of 30-day months to demonstrate the New Testament fulfillment of Daniel’s prophecies about the Messiah. (See excerpts from his book here.)  In Revelation, God’s two witnesses prophesy for 1260 days, which come to exactly 42 biblical months or 3½ biblical years.

*Ben-Dov, J. (2021). A 360-Day Administrative Year in Ancient Israel: Judahite Portable Calendars and the Flood Account. Harvard Theological Review, 114(4), 431-450.

For links to pages about biblical holidays and holy days, click here.