Summary of Calendar Calculations Theory.

To understand how the Biblical Calendar works, we have to look at just a few key points.

Once we have the first day of the first month, the Holy Days can be inserted into the calendar as instructed by God in Leviticus 23.

The instructions start in,

Gen 1:14 NET.  God said, "Let there be lights in the expanse of the sky to separate the day from the night, and let them be signs to indicate seasons and days and years,

Therefore, the Biblical calendar requires us to use the positions of the Sun, Moon and Earth in our calculations.

As the earth rotates around the Sun, there are four key points in our observation of the Sun. They are the longest and shortest days, and the two equinoxes (the days and nights are equal length, using the center of the Sun for the calculations. Daylight and dark are not equal because the starts with the leading edge, and ends with the trailing edge).

The Hebrew word that points us to these four seasons is found in,

Exo 34:22 NET.  "You must observe the Feast of Weeks — the firstfruits of the harvest of wheat — and the Feast of Ingathering at the end of the year.

Exo 23:16 NET.  "You are also to observe the Feast of Harvest, the firstfruits of your labors that you have sown in the field, and the Feast of Ingathering at the end of the year when you have gathered in your harvest out of the field.

The word translated "end" in Ex34:22, comes from the Hebrew word #8622 (tek-oo-faw, or tekufar) and means circuit, or turn of the year. Understanding this word is a key point for understanding the Biblical calendar.

Imagine a man standing on the equator facing West with two cymbals in his hands. At the Northern Hemisphere (NH) Spring equinox the cymbals are together. As the sun moves North, the cymbal in his right hand follows the sun, and moves out until the longest day is reached. It then moves towards the other cymbal until the Autumn equinox is reached. At this point the left hand cymbal moves out until the NH shortest day is reached. To complete the circuit, the two cymbals come together and we are back to the starting equinox. The sun has completed it's "circuit".

Why is "tek-oo-faw" so important?

Because it tells us that the Feast of Tabernacles (Feast of Ingathering) should not start until after the Autumn equinox.

To ensure that this will always happen, the new year (Ex 12:2 and Ex 13:4 = Spring) can not start until after the NH Spring equinox.

The Jewish or Hebrew calendar often breaks this rule, and therefore should not be used by Christians who want to live by "the words from God's mouth" (Mat 4:4).

Some definitions.

Start of a day.

The normal day starts at the published sunset time for your location, or nearest city. However, as I have mentioned elsewhere, the sun can sometimes still be visible for up to seven minutes after the calculated sunset time. To avoid giving people who do not understand this, the wrong impression, it is best to stay in "Sabbath mode" till the sun has visibly set.

For calculation purposes, we use an "average" day, or a day that always starts at 6:00pm (18:00), at the IDL.  This standard sunset time is also used in the Hebrew calendar, and because sunset is always after 6:00pm at the Spring (NH) equinox, using an average day does not cause any errors in finding the start of the year (but does simplify the calculations).

Remember, to find the time at the UTC point, you have to add 12:00 hours to the IDL times (IDL and UTC are 180 degrees apart). Thus, the average day sunset of 6:00pm at the IDL is  6:00 am in UTC time.

Start of a month.

Since the new moon can occur at any time throughout a day, it makes sense to me to call the first day that everyone around the world can be under a new moon, the first day of the month. For our calculation purposes, a calendar day starts when the “average day” sun sets at the International Dateline. A month starts the first day that there was a New Moon in the previous 24 hours at the “average day” sunset of 6:00pm at the International Date Line.

Start of the year

The same logic is used for the first day of the year. The year starts with the first day of the first month after the Spring equinox. Taking our six hour adjustment into account (sunset not midnight) the day after the day of the Northern Hemisphere Spring equinox is the start of Spring, and the earliest that the first month of the year should start. Under these rules, each new year will always start somewhere within the first 30 days of Spring. Since we look at what the situation is at the start (6:00pm at IDL) of a day, in the rare case of where the equinox and the new moon conjunction both occur in the previous  (sunset to sunset) 24 hour period, then the next day is both the new month and new year. Which is the case in 2015.

Calculating the Calendar.

Before we can pencil in the Holy Days, we need a basic calendar. This can be a normal calendar with enough room to write on, or we can start with just a list of all the important dates, and transfer them over later. The place to start is to first find the March equinox and the times of the New Moons for the year. Make sure that any times given are in Universal Time (abbreviated as “UT” or “UTC”). There is less than one second difference between UT and UTC, so for our purposes either can be used. These dates and times can be found from published charts and from the Internet (e.g. U.N. Naval Observatory [], Geoscience Australia, Nasa,  [scan down to find UTC times in the RH column]). The times for the New Moons on wall calendars are usually local times, so corrections have to be made to bring them back to UT, and that can make the calculations more complicated.

This is how I do it, but there may be other ways of doing the calculations.

To find the moon phases.

Go into

It will usually come up with the current year. To get next years phases, scroll down past the first block of dates and you will see where you can move the year forward or backward. Set the year you want, and let it recalculate.

Again scroll down to the second block of dates and times and UTC is on the RH side of the block.


Key in the first day of the year. Number of phases - try 50.  Format - display by phases. Then click on "get data".

To find the equinoxes.

Now you have all the information you need.

To make the calendar.

Take the date of the two equinoxes and write them on the calendar. Just for interest, I pencil in the longest and shortest days as well.

Look at the date and time for each of the new moons. If the time is less than 6:00 hours UTC, take the date given and mark it on the calendar. If the time of day is 6:00 hours UTC or greater, increase the date by one, and mark it on the calendar. This is the start of each month.

When all the new moons are done, look for the closest new moon after the March equinox (can't be the same day). That is the first day of the new year for God's calendar.

Count down 14 days, and mark the evening before as the Last Supper. The 14th is the day the Passover lambs (and Christ) were killed. The 15th day is the first Holy Day of Unleavened Bread. Seven days later (21st) is the Second Holy Day of Unleavened Bread.

From the Sunday (the day after the Sabbath) during Unleavened Bread, count 50 days (seven full weeks plus one day), and that Sunday is Pentecost. 

The first day of the seventh month is Trumpets, followed by Atonement on the 10th day, and Tabernacles starting on the 15th day, and Last Great Day on the 22nd day.

Make a note at the top that the actual Feasts start the evening before at sunset.

Now you have God's Holy Days marked on a calendar.

The original article with more background as to how I came to learn how to extract God's calendar out of the Bible, can be found here.

Bob Orchard April 2014

While this information is made freely available (Mat 10:8b) , and can be printed out, it is done with the understanding that there will only be fair and honest use of the material, and that it will be copied in full with no alterations.