The most fundamental building block of any calendar is the day, so before we can determine anything else we must identify where and when the day begins. Tradition tells us that the day starts at sunset and we will explore those claims in the chapter When Does the Day Start? because it is a less fundamental issue. The real fundamental issue is "where" does the day begin because whether sunset or first light, not every place on earth experiences it at the same time.

Once we know where the day begins, then we can discuss what signs, if any, start the year, month, sabbath week, and day. The question of "where" is often dismissed as a "corner case" or an "exception" that has no practical relevance, but it is actually the most fundamental question for anyone claiming they observe a Sabbath that goes back to creation or are keeping feasts on the proper day where they live. Without agreeing on this starting meridian dateline, timekeeping devolves into local solipsism: your "today" could be someone else's "yesterday" or "tomorrow" just a few hundred miles away.

This isn't an edge case—it's the core calendar design problem that time zones and the International Date Line (IDL) were invented to solve. The problem is all fixed datelines have been invented within the past 1000 years and the IDL is less than 180 years old, yet we are searching for objective and divinely authoritative definition of when and where one day begins and another ends for everyone on earth. Picking what feels good or is convenient or familiar isn't good enough. We want truth, not tradition.

There are multiple proposed locations with various theories about how the Bible could imply it, but the exact location is of far less significance than whether a fixed location is even possible in the first place given the principles laid out to evaluate calendars. If the location is fixed, can everyone on earth know when the year or month starts even if they cannot observe the sign from their location?

According to Jewish tradition a day begins at sunset; therefore, people presume one can simply count sunsets in order to keep track of the Sabbath. A calendar that counts days by observing the sunset falls apart the moment people start migrating around the globe. Those who migrate east will count a different number of sunsets from those who migrate west; therefore, when their descendants meet on the other side of the world they will not have an objective definition of the 7th day Sabbath. One people group will be a day ahead, the other a day behind and there would be no objective way for both to keep a continuous 7 day cycle together.

When Magellan sailed around the world his crew kept detailed logs of every day of their journey, yet when they arrived home they discovered their date differed by one day from those who stayed home; therefore, a devout Saturday Sabbath Christian or Jew who sailed around the world would end up keeping a different Sabbath day than everyone else and he wouldn't even know it until he came back to land. He could easily spends months observing the wrong sabbath for his timezone if he relied on his own subjective count of sunsets.

The historical record from Antonio Pigafetta's journal (the primary eyewitness account) explicitly states that when the Magellan's ship reached the Cape Verde Islands, the crew believed it was Wednesday (July 9, 1522, according to their logs), but the locals informed them it was Thursday, July 10th. Pigafetta wrote:

They answered it was Thursday, at which they were much amazed, for to us it was Wednesday, and we knew not how we had fallen into error.

— Antonio Pigafetta, Journal

If the day count is tied to the land and not the observer, then an international dateline is required and each person would be responsible for knowing when they crossed it in order to keep the Sabbath appropriate for their location. The modern international date line was invented in 1844 (only about 180 years ago). This dateline is not fixed and countries can move it by decree if it suits their economic interests to do so.

The Island of Samoa caused a great disturbance among the Seventh day Adventists church when they passed a law moving the country to the other side of the international date line. With this change those who wanted to stay on the seven day cycle they have always known would have to start worshiping on Sunday instead of Saturday. No one ever stopped to ask how they knew that the original dateline was correct in the first place.

Modern Jewish scholars interpreted the following passage from the Babylonian Talmud, Rosh Hashanah 20b, to determine a dateline:

Rabbi Zeira said that Rav Naḥman said: The moon is invisible for twenty-four hours [around the conjunction]. For us [in Babylonia], six hours of the old [moon] and eighteen of the new [moon]. For them [in Eretz Yisrael], six of the new [moon] and eighteen of the old [moon].

— Babylonian Talmud, Rosh Hashanah 20b

This is interpreted to mean that the moon's invisibility spans 24 hours globally, with the division (6/18 hours) reflecting time differences between regions. The Talmud further states that if the molad (the average time of dark moon conjunction) occurs before noon in Jerusalem, the moon might be visible close to sunset somewhere, but if after noon, it won't be—implying a "cutoff" point where the day has fully begun worldwide.

We calculate its [the moon's] birth [molad]. If it was born before noon — it is certain that it [the new moon] will be seen close to sunset. If it was born after noon — it is certain that it will not be seen close to sunset.

— Babylonian Talmud

Baked into the Talmud quote is an assumption "The moon is invisible for twenty-four hours" and yet Stellarium reports that the moon is 0.0% illuminated for just 6.5 hours and after 12 hours it is just 0.2% illuminated. Modern astronomy shows that it takes at least 15 hours from the conjunction to be visible with the naked eye, an implied 0.4% illumination and a total period of 30 hours of "invisibility". Furthermore, the alleged "24 hours" of invisibility would rarely align with any fixed dateline, aligning only about once every 2 years.

This is just one conception of the dateline, another is to view Jerusalem as the center of the world and the dateline as 180 degrees on the other side of the globe.

Thus says YHWH: This is Jerusalem. I have set her in the center of the nations, with countries all around her.

— Ezekiel 5:5

And with this perspective added to the mix, there are now three potential datelines that can be used and all of them "conceived" within the past 1000 years.

Synchronizing The Northern and Southern Hemispheres

But there is one additional wrinkle, the sign you use to start the month might not be seen the same way in northern and southern hemispheres and thus the dateline issue becomes far more complex for dark and sliver moons than for the full moon. A problem with the sliver moon is that there are often over 24 hours difference between when the first visible sliver can be seen in the Northern and Southern hemispheres. This discrepancy arises from the Moon's orbital tilt relative to the Earth's equator (about 5 degrees), combined with the seasonal variations in the ecliptic's angle to the horizon. At the crescent phase, the Moon is only visible for a brief window (typically 15–60 minutes after sunset), and its low position makes it highly sensitive to latitude: northern observers might see it one evening, while southern ones at the same longitude wait another day due to the Moon's apparent path appearing steeper or shallower.

This becomes obvious at the extremes of the North and South poles, where the 24 hours of darkness during part of the year means the sliver isn't visible at all—since the sliver is only observable when the Moon is near the Sun, requiring a clear post-sunset horizon. Meanwhile, other phases of the Moon, like the full or gibbous, can be visible in broad daylight or during polar twilight.

But this isn't just a polar issue—it affects moderate latitudes too, where most people live. For example, at latitudes around 30–40°N (e.g., the Mediterranean or southern U.S.) vs. 30–40°S (e.g., southern Africa or Australia), the crescent's visibility can differ by a full day due to the Moon's position relative to the horizon and atmospheric conditions. This can cause two people at the same longitude to be on different calendar days because the sliver is visible for such a short period of time in a given night that it could easily be below the horizon, obscured by haze, or too close to the Sun's glare in one hemisphere but not the other.

Real-world examples from Muslim communities, who rely on crescent sightings for starting Ramadan and celebrating Eid, illustrate these challenges vividly. In 2025, controversy erupted when Saudi Arabia (in the Northern Hemisphere) announced the Eid al-Fitr crescent sighting on March 29, despite astronomical data showing it was scientifically impossible—due to the Moon being too young (less than 12 hours old) and a recent solar eclipse making visibility improbable for hours after. This led to accusations of premature declaration, with southern countries like Australia and Indonesia often starting a day later based on local sightings, causing global disunity. Similar issues recur annually: Northern Hemisphere nations (e.g., UK, Morocco) might align with Saudi announcements, while Southern Hemisphere ones (e.g., South Africa) report no sighting, resulting in different Ramadan start dates even in the same time zone. These discrepancies aren't rare—they stem from the inherent variability of crescent observation, leading to "moon wars" that frustrate communities, disrupt family gatherings, and even impact economies (e.g., varying holiday dates across borders). Scholars debate "unity of horizons" (one global sighting binds all) vs. "difference of horizons" (local/regional), but the practical result is ongoing division.

In contrast, on a full moon calendar, northern and southern hemispheres are synchronized by longitude in all regions that have at least some short period of darkness every night—which covers the vast majority of habitable Earth (up to about 60° latitude). The full moon is visible for nearly 12 hours (rising around sunset, setting around sunrise), making it far less dependent on brief windows or perfect horizons. It's observable in daylight if needed, unmistakable in shape, and aligns naturally across hemispheres, fulfilling the biblical call for signs that are accessible and unifying for all peoples under heaven (Gen. 1:14; Deut. 4:19). This avoids the disunity and subjectivity plaguing crescent-based systems, ensuring God's appointed times remain objective and decentralized.

How to Test Datelines?

How then can we test these datelines to determine the truth? After all, one cannot simply pick the one that "feels" the best or "appears" most logical to our human mind. We are seeking God's truth not man's concept of the best system.

In addition to the Sabbath we need to know the day a new month begins and scripture tells us that the sun, moon, and stars are to be the signs of the appointed times. The problem is all of these fixed datelines rarely sync up with the moon's visibility because it takes 29.530587 days for the moon to circle the earth. This means that each month the location of the conjunction (or other sign) drifts by ~191.01 degrees on the earth's surface. If there were to be a dateline then one could easily argue that it would have to move with the sign of the moon and by doing so the calendar remains tied to observation of the sun, moon, and stars.

If the dateline does not move with the moon, then everyone, everywhere would have to calculate whether the sign of the start of the month was hypothetically visible in Jerusalem (or anywhere else in the world deemed to have Biblical authority for signs). The idea of the "dateline" moving with the moon may seem foreign, but it is just a formalization of what up to a billion people already do when you factor in the Muslim population, Jews that use local observance, and even many in the Hebrew Roots movement.

Solar only calendars, such as presented in the Book of Jubilees, don't have the moon sighting to work with; therefore, they must use something like the equinox at the start of the year, this means either their "dateline" would rotate about 87.192 degrees on the surface of the earth every year (0.2422 days) or everyone must calculate the equinox in Jerusalem and use one of the other fixed datelines.

Observation vs Calculation

There is only one way I know to solve the mystery of the dateline that preserves the property that the calendar is knowable by observation and not calculation: a dateline that is implied by the movement of the sun, moon, and stars and not fixed to any geographical location. Each location makes its own observation of the heavenly lights in the sky to determine when the year, months, and calendar days begin.

And God said, 'Let there be lights (plural) in the expanse of the heavens to separate the day from the night. And let them (plural) be for signs and for seasons (appointed times), and for days and years, and let them be lights in the expanse of the heavens to give light upon the earth.'

— Genesis 1:14-16

Any fixed international dateline is using something other than the lights in the heavens to separate days and appointed times for everyplace on earth except Jerusalem. It would require sophisticated calculations and/or communication networks to implement globally.

The Jews use the molad to determine when the sliver moon is hypothetically visible. The molad is a mathematical average time of the lunar conjunction (when the Sun, Earth, and Moon align, making the Moon invisible) and only accurately predicts a hypothetically visible crescent about 80% of the time. The end result of their approach is that they are not actually using the "lights in the expansion of the heavens" to determine the appointed times, but rather the absence of light and a poor approximation derived calculation from an average.

To maintain a fixed dateline requires everyone, everywhere, to agree on a fixed point of reference for observing the lights in the heavens given by God to define signs, and seasons (months), and days and years. Most would suggest Jerusalem as the "center of the nations", which, in theory, would make Jerusalem the reference timezone and place the dateline 180 degrees on the other side of the globe; however, it doesn't matter where you pick if it isn't possible to trivially calculate or communicate it to every location on earth in a timely manner.

With these two things (location and celestial sign) established a person would require the ability to calculate if it were hypothetically possible to view the visible sign in Jerusalem. Calculating the visibility of a thin sliver moon (young lunar crescent) in Jerusalem from any random location on Earth — using only pen and paper in the pre-computer age — would be extremely difficult and, for all practical purposes, impossible to do accurately from distant locations.

Core Challenges with Pen & Paper

No precise ephemerides (datasets) — You need the exact time and position of conjunction (sun, moon, earth alignment), plus the daily motions of Sun and Moon. In ancient/medieval times, these were approximated from hand-calculated tables (zīj in Islamic astronomy), which were themselves laboriously derived over years and copied by hand. Without a good zīj for your latitude, you're basically guessing.

Topocentric correction is brutal — Visibility depends on positions as seen from Jerusalem's specific latitude/longitude, not Earth's center. The Moon's parallax is ~1° max, so from a random location (especially far away), you need to adjust for your own position relative to Jerusalem. This requires spherical trigonometry, knowing your exact latitude, and estimating the geocentric → topocentric shift — all by hand with log tables or sine tables if you're lucky.

Key parameters are tedious to compute:

• Moon's age at Jerusalem's sunset (hours since conjunction)
• Lag time (time between sunset and moonset in Jerusalem)
• Elongation (angular separation Sun–Moon)
• Altitude of Moon above horizon at sunset
• Azimuth difference between Sun and Moon

Each requires iterative calculations using mean motions, anomalies, equation of center, etc., from Ptolemaic/Indian/Islamic models — hundreds of multiplications/division by hand.

No global view — From a random spot on Earth, you can't easily know if the Moon is even above the horizon in Jerusalem at the critical time. You'd need to know Jerusalem's local time relative to yours (longitude difference × 4 min/degree), which again requires tables or very rough estimates.

Determining longitude 1000 years ago (around the 11th century AD, during the Islamic Golden Age and early medieval Europe) was extremely difficult — one of the hardest problems in astronomy, navigation, and cartography at the time. It remained unsolved for practical, everyday use (especially at sea) for another 700 years, until the 18th century with marine chronometers (mechanical clocks).

The core principle was already understood since ancient times (Hipparchus, ~150 BC): longitude difference = time difference × 15° per hour (since Earth rotates 360° in 24 hours). But turning that into accurate measurements required knowing the exact same moment at two distant places — essentially synchronizing clocks across vast distances without modern technology.

The only practical method in the ancient world is the simultaneous observation of lunar eclipses which occur at the same time on all parts of the world. This may work for pairs of cities, but accuracy depends upon the precision of measurements and in ancient times this was lacking.

For all intents and purposes it would be impossible for the average person to determine and validate the times and seasons. Even if they had all the raw data, exceptionally few had the skills to perform the calculations. Astronomy required wealthy individuals to hire you, give you access to rare texts and tools, or ample leisure time — privileges of the elite. What is more, only about 5% of the population is mentally gifted to the degree required to learn the calculations. If there was no one around to train you in how to perform the calculations only 1 in a million people would be able to derive the formulas from first principles.

The masses would be at the mercy of an elite group of people to determine a local calendar that depended upon calculating observations that would have been made in Jerusalem. For a short period of time the Jews claim that signal fires were used to communicate observations from Jerusalem to Babylon; however, that system didn't last long because the Samaritans would light false fires. In other words, this kind of calendar system fundamentally ties the masses to an elite group of leaders who get to determine the calendar and who ultimately manipulate it for political purposes.

…the pontiffs misused this system of intercalation horribly. For they, being men, and politicians too, used to intercalate for the advantage of tax-farmers or against the interests of defendants in pending cases, so that festivals and sacred days might fall at particular times to suit their convenience.

— Censorinus, De Die Natali (On the Birthday), 3rd century AD

Julius Caesar, having been elected Pontifex Maximus in 63 BC (a position that gave him ultimate authority over the calendar), used the chaos as justification for radical reform. In 46 BC he decreed the famous "Year of Confusion" (445 days long) to realign the calendar with the sun, then introduced the new Julian calendar starting in 45 BC (365 days with a leap day every four years), effectively removing human discretion over months and intercalation entirely. The reform deliberately broke the old pontifical monopoly that had enabled the political manipulation.

The astute individual might notice that the Jews have used a calculated calendar going all the way back to 350 AD. The problem is this calculated calendar is only an approximation and not an honest, objective, reflection of the sun, moon and stars and it certainly doesn't allow those in distant lands to use an objective light in the heavens as the sign of the appointed times.

In summary, an international dateline is required to overcome the inherently subjective nature of counting observed sunsets and a fixed international dateline is impractical to implement by observation alone in ancient times. This means that any calendar based on it requires extensive measurement precision followed by high precision computation or speed of light communication and these are things which have only existed for the past 200 years or less.

The Bible informs us that the lights in the heavens are to be used to determine the appointed times. I interpret this to imply that the average person should be able to determine the times simply by observing the sky where ever they happen to be. This implies that the international dateline moves with the sun or moon and that fact alone means a continuous cycle going back to creation is extremely unlikely to be what someone only reading the written scriptures could conclude and, most importantly, be able to obey.

Objections

Critics may object that a moving celestial 'dateline' lacks explicit Scripture. Yet the same charge applies equally to any fixed meridian—whether secular (IDL, 1884) or halakhic (Jerusalem-opposite proposals from medieval rabbis onward). No verse commands, 'Establish a permanent line 180° from Jerusalem to preserve the week,' nor narration demonstrate such a mechanism. Scripture is silent on global synchronization, focusing instead on local obedience via heavenly signs (Gen. 1:14). Thus, both approaches infer beyond the text, but only local observation adheres strictly to 'no additions' and 'accessible to all' principles without requiring elite calculations or modern tech.

The definition of the dateline impacts all of the feast days, because the first day of the month is based upon the day a particular sign is observed and it impacts the continuous cycle because the geographical definition of a day changes with the moon. Therefore, the Lunar Sabbath theory may be the most aligned with the practical realities of a calendar where days are determined by the lights in the heavens and not merely by our individual subjective counting of sunsets.

But we will not rest this conclusion on this unavoidable mathematical reality of the earth and moon having orbits that are not evenly divisible by a 24 hour day. This book will go on to prove via many independent lines of evidence that the cross was not in 30 AD and was almost certainly 32 AD. This would produce a Monday Passover on the traditional calendar, which is totally incompatible with the narrative in the gospels. This and many other lines of evidence confirm what we just inferred must be true from first principles of astronomy.

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All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident.

— Arthur Schopenhauer