Tracking Time: Where Do We Get Our Calendar

The answer’s in Genesis (and Exodus)

by Liz Abrams on October 4, 2022

October 4 is the anniversary of the institution of the Gregorian calendar, by which most of the world keeps track of what day, month, and year it is. The Gregorian calendar was instituted in October 1582 to replace the Julian calendar, which had been the calendar system for over 1600 years. The Julian calendar is still used in a few places in the world. If we were still using the Julian calendar, October 4, 2022, would instead be September 19!

One of the most fundamental ways we understand events is by when they happened. Your birthday comes around once a year. Christmas is always December 25, and Valentine’s Day is February 14. You might reflect that it has been ten years since a significant life event, and couples who get to celebrate a 50-year anniversary are seen as a picture of an exceptionally faithful marriage.

But where did calendars begin? Adam wasn’t created on January 1, was he? And even though we celebrate Jesus’ birth on December 25, few people would suggest that as the actual day he was born. As with many things, the origins of the calendar go back to Genesis.

The first thing mentioned in Genesis that would indicate time passing in a way we could measure is the day/night cycle that began on the first day when God separated the light from the darkness. Even though the sun and moon had not yet been created, somehow there was a time when it was light (presumably as it would be experienced by a person on the earth) followed by a time of darkness. That they are named day and night would indicate that they were of comparable length to the day and night we experience today with the sun and moon.

On day four, God created the sun, moon, stars, and other astronomical objects. At the same time, he stated their purpose.

And God said, “Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons, and for days and years, and let them be lights in the expanse of the heavens to give light upon the earth.” (Genesis 1:14–15)

The sun, the “greater light” gives us the obvious division of day and night. The moon gives us the next division of time—the month. The moon reflects sunlight, and when the half of the moon reflecting sunlight faces the earth, that is a full moon. When sunlight reflects off the half of the moon not facing the earth, that is a new moon, and in between are various phases of waxing and waning. From new moon to new moon is a lunar month. Even though today’s calendar in the Western World does not line up with the lunar months, ancient calendars did.

There are a few ways to measure the year using the relationship between the sun and the earth. A solar year is the time from one vernal equinox to another—when the sun crosses the celestial equator. A sidereal year measures the earth’s orbit around the sun with the stars as its reference frame, and it’s approximately 20 minutes longer than the solar year. An anomalistic year is the time it takes for the earth to return to the point in its orbit when it is closest to the sun, and that is about five minutes longer than the sidereal year.

Ancient Calendars

It is unclear exactly when people started using calendars. Adam’s age is measured in years, and we believe that is an accurate measurement, which means he must have figured out how to measure time fairly early. But which years was he using? Months and days do not neatly fit into the solar year—a lunar year of 12 lunar months is 11 days shorter than a solar year. If he was using lunar years (12 lunar months = 354 days), he would be 901 years old in solar years. If the 1656 years from creation to the flood was measured using lunar years, that would be 1606 solar years.

A careful examination of the timeline given in Genesis 6–7 reveals that Moses reckoned the flood duration using a lunar calendar of 360 days divided into 12 months of 30 days. See Biblical Overview of the Flood Timeline for a detailed explanation.

The earliest evidence we have outside the Bible suggests that people became aware of the difference between lunar and solar years very early and came up with lunisolar calendars, where the months were lunar but the years were solar. In Sumer, intercalary months were periodically proclaimed by kings to bring the calendar back into line with the solar year, but this was done fairly haphazardly until Persian astronomical knowledge made it possible for them to standardize intercalations.

Ancient Egypt had a calendar of 365 days that lined up fairly well with the solar year—12 months of 30 days each plus an intercalary month of 5 days. This calendar meant that the Egyptians lost about a day every four years.

When God Reformed the Hebrew Calendar

Just before the Passover seems like an odd time for God to address the Hebrew calendar. He tells Moses to have the Israelites plunder the Egyptians by asking for all sorts of precious items and to eat the hastily prepared Passover meal with their sandals on because they needed to be ready to leave at a moment’s notice. And this (the month of Abib, later called Nisan, which would roughly correspond to March/April for us) was to be the first month of the year for them (Exodus 12:1–2).

God didn’t waste any time disassociating Israel from the false gods of the Egyptians, right down to changing the calendar.

Why did God choose that moment for this calendar reset? The death of the firstborn culminates a series of judgments where God not only judged Pharaoh and Egypt for the persecution and slavery of the Israelites but also demonstrated his supremacy over the false gods of the Egyptians. These false gods were intertwined with the calendar. God didn’t waste any time disassociating Israel from the false gods of the Egyptians, right down to changing the calendar.

Instead, the Hebrew calendar would be defined by God and his acts on behalf of Israel. His deliverance of Israel was so paradigmatic for how they were to see him that the month during the exodus defined the beginning of the year.

Sabbath Principles

At the same time, God reformed their week. While God created in seven days, we don’t have explicit mention of a seven-day week for people based on creation week until God instituted the Sabbath. Several seven-day periods during the flood narrative indicate that people may have had a seven-day week prior to the Mosaic Law, but several cultures had “weeks” that were different lengths. Egyptians had a 10-day week, but God’s instructions for the Passover assume a 7-day week. Some point out that Sumer had a 7-day week based on the amount of time it takes the moon to change from one phase to another and that the Babylonians inherited a 7-day week from them. But this early in history, Israel did not have significant interaction, let alone cultural exchange with societies that far away.

If the Israelites had been observing Egypt’s 10-day week, God’s institution of the Sabbath would cement a 7-day week for the nation. When God gave the Israelites manna, he told Moses to instruct the Israelites to gather twice as much on the sixth day in order for the seventh to be a Sabbath for them. The people were ordered to rest on the seventh day, but the true significance of the Sabbath would not be explained until Sinai where God gave the Ten Commandments. He explicitly linked the Hebrew observance of the Sabbath to God’s rest after the six days of creation (Exodus 20:8–11). Since Moses was the human author/compiler of Genesis, it is possible that Israel had forgotten about the history of creation, especially being surrounded by Egyptian mythology for hundreds of years. We don’t know how much of the history we have in Genesis was remembered in the general Israelite population until they received it back in the form of God-inspired Scripture.

The observance of the Sabbath for ancient Israel was so important that God instituted the death penalty for breaking the Sabbath in Exodus 31 and again in Exodus 35. It was to be a solemn reminder of the covenant between Israel and God and a continual reminder that God created the world in six days and rested on the seventh day.

Just as Israel was to observe the weekly Sabbath, they were commanded to release Israelite servants after they had served six years and to let fields go fallow in the seventh year (Leviticus 25). These customs echoed and reinforced the cycle of rest after six periods of work, but they did not replace the primary pattern of resting on the seventh day.

Annual Festivals

God also ensured the Israelite year would revolve around festivals worshipping him, not the false gods worshipped in Egypt and Canaan. The Feast of Unleavened Bread at the beginning of the year, the Feast of Harvest, and the Feast of Ingathering at the end of the year were festivals where Israel could worship and celebrate God’s deliverance and provision (Exodus 23:14–17). These festivals were so important that God reiterated them in Exodus 34. If the emphasis is on replacing pagan practices with the explicit worship of the true God, it also explains an otherwise-odd prohibition on boiling a goat in its mother’s milk. Rather than a culinary practice, it was probably a ritual meant to ensure the land’s fertility. Instead of relying on such pagan practices, Israel was instead to trust God and celebrate the Harvest Feast at the beginning of harvest and the Feast of Ingathering at the end of the harvest.

Israel, however, corrupted the observance of these feasts through their idolatry. In Isaiah’s day, God even repudiated the feasts he instituted.

When you come to appear before me, who has required of you this trampling of my courts? Bring no more vain offerings; incense is an abomination to me. New moon and Sabbath and the calling of convocations—I cannot endure iniquity and solemn assembly. Your new moons and your appointed feasts my soul hates; they have become a burden to me; I am weary of bearing them. (Isaiah 1:12–14)

The frequent religious festivals in the Hebrew calendar and weekly Sabbath rest were meant to be a continual reminder of Israel’s special relationship with God. But when Israel fell into idolatry, their observance of these commanded festivals was hypocritical and ran contrary to God’s original intention for them.

Turning Calendars into Chronology

What calendar you use determines how you measure the year. But how did ancient people measure the passage of years? The global flood happened around 1656 years after creation, but Genesis doesn’t say, “1656 years after creation.” It says, “When Noah was 600 years old,” so events may have primarily been tracked by how old a given patriarch was when they happened. This also explains the existence of the Genesis 5 and 11 genealogies containing the ages of the patriarchs at the birth of the next in line and their total lifespan—these would be necessary details to turn, say, “The 600th year of Noah’s life” into “1656 years after creation.”

After the flood and the dispersion in Babel, at least some places rather quickly developed a culture and government system with a reigning king. It is not surprising that the system in those places shifted from reckoning time by the lifespans of patriarchs to marking time by the reign of kings. Sumer, Babylon, and Egypt all used regnal years. Documents like the Sumerian King List and Canon of Kings provide evidence of this practice. It is significant that Genesis provides a tight chronogenealogy up until the point Jacob entered Egypt at 130 and died at 147. In Egypt, the Israelites would have encountered the regnal year calendar and perhaps also gradually stopped reckoning time by the lives of patriarchs. While the Bible provides ample chronological markers to accurately measure time from creation to the Jewish return from exile (and thus the age of the earth), after Jacob’s death the markers are not primarily the lifespan of a given individual.

The Ancestors of the Modern Calendar

The ancestor of the calendar we use today is the ancient Roman calendar in which months started with the new moon and must have included intercalations to bring it into alignment with the solar year. Legends said that Romulus instituted the first calendar of 304 days divided into 10 months. It is unlikely that this calendar was historical because it would immediately be out of alignment with the solar year. There is also no historical evidence of this calendar actually being used.

Instead, the earliest attested calendar—the Roman republican calendar—had many similarities to the modern calendar. It had twelve months, many of them named similarly to the months we mark today. The length of the year varied over a 24-year cycle due to the inclusion of an intercalary month at certain intervals. This intercalation, however, resulted in a quite accurate average year length of 365.25 days.

Julius Caesar reformed the calendar in 46 BC to have a normal year length of 365 days and a leap year every fourth year of 366 days. That is a simpler way of achieving a year of 365.25 days—but it is also slightly longer than the actual solar year, which is 365.24219 days. This means that the Julian calendar gains a day every 128 years. This may not seem like a significant drift, but over time it added up.

The Gregorian calendar slightly changes the nature of leap years to bring them into even closer alignment with the solar year.

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.1

Calculating the Age of the World

At the time of writing, it is 2022, but 2,022 years from what? The abbreviation AD stands for the Latin anno Domini, which means “year of our Lord.” It intends to mark how many years have passed since the birth of Christ, but for several reasons, it’s a couple years off. Most historians would place Jesus' birth somewhere between 6 and 2 BC.

Because we have biblical records that allow us to measure how many years passed between creation and the time of Jewish return from exile, at which point we can match up the biblical timeline with the secular timeline, it’s possible to calculate the age of the world with a high degree of accuracy. However, the records do not make it possible to calculate the age of the world down to the day, or even the exact year, given the inherent uncertainty in the measurements we are given. We don’t know whether Seth was born on Adam’s 130th birthday or any day between that and the day before he turned 131. And was Adam’s age inclusive or not—i.e., on the first anniversary of his creation, was he counted as one year old or two? Similar uncertainty happens at every link on the chronological chain, adding up to a few years—but not decades—of uncertainty regarding the exact age of the earth.

Various Jews and Christians throughout history who took the Bible’s history seriously have used the Old Testament to create a chronology of the world. Archbishop James Ussher is the most famous one, and his Annals of the World is fascinating reading to this day. He believed the earth was created on October 23, 4004 BC. Isaac Newton is best known for his scientific genius, but he wrote more about theology than science and also calculated the age of the earth. He believed the world was created in 3998 BC. These men, and others who calculated the earth to be a similar age, were among the premier scholars of their day.

What Lessons Can We Learn?

Even though the world is fallen, the sun, moon, and stars still fulfill their created purpose in marking the days, seasons, and years. When our calendars fall out of line with the sun, our calendars must change to fit the solar year, not the other way around.

God created the world with a way to mark time and history in mind, and the Bible records enough historical details to allow us to measure the history of the world. Just as God created an orderly universe that allows for scientific experimentation, the order of the heavens allows for historical investigation.

Just as God gave the Hebrew people the Sabbath and festivals to continually remind them of their special relationship with him, most Christians historically have worshipped on the first day of the week to celebrate Jesus’ resurrection. Some still worship on Saturday to commemorate the Sabbath that God originally instituted. The day that we worship is not as important as the fact that we do worship. It is also common to commemorate Jesus’ death during Easter and his birth during Christmas. While these are never commanded in the Bible, they fit with the theme of remembering God’s goodness to his people throughout the year.


  1. “Introduction to Calendars,” Astronomical Applications Department, US Navy, accessed October 3, 2022,


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