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Are there really 13 months in a year instead of 12? (13597 hits)

Are there 13 months in a year? A secret month omitted from calendars as far back as 45 B.C?

The 13th Month Theory will attempt to answer this question. You will find in this article a brief overview of current and past calendar systems, a mathematical equation supporting the theory that a 13th month may currently exist, but was likely omitted and forgotten in centuries long past. So stick around for a while. It’s all conjecture, but it gets interesting towards the end….

The Gregorian Calendar is the one of the most commonly used calendars in the world. There are 12 months in the Gregorian Calendar as listed below:

However, January wasn’t always the first month in a calendar year.

In fact, on the Julian Calendar, which was calculated and organized by Julius Ceasar but implemented after his death in 45 B.C., March was listed as the first month of the year.

April was the second month,
May was third,
June was fourth,
July was fifth ,
August was the sixth month (formerly known under its Greek name; s*xtiliss),
September was the seventh month,
October the eighth month of the year,
November the ninth month,
and December was the tenth month of the year.
The “Sept” in September refers to the number 7 in latin, the “Oct” in October refers to the number 8, the “Nov” in November refers to the number 9, and the “DEC” in December corresponds to the prefix of the number 10 in latin.

So how did the latter months come to be known as the 9th, 10th, 11th, and 12th, instead of the 7th, 8th, 9th, and 10th, respectively?

On the Julian calendar, the months January and February were the last two months of the year and not the first and the second, unlike the Gregorian Calendar.

A Gregorian year is approximately 365 days, with a leap year every 4 years [that cannot be divided by 100 or 400]. A Julian Calendar is 365.25 days a year, with a leap year added every four years in February.

The Julian Calendar was a commonly used calendar until Pope Gregory XIII approved an updated version of the Julian Calendar in 1584 - referred to as the Gregorian Calendar.

You may wonder how this all ties in to having a 13th month, or a “missing” month in a standard Lunar year.

Under our current calendar, there are twelve months. The definition of a month is the period in which the moon leaves one point, to orbit around the Earth in a gravitational cycle that would cause it to arrive at the origional point again. Generally a lunar cycle (or month) averages 27.9 days. A Synodic month has 29 days. Since the dawn of humanity, humans have used the moon to calculate the seasons and time.

To begin, here’s a bit of Q. and A. (Trust me, this is going somewhere)

Q. How many days are in a week?

A.) There are 7 days in a week.

Q. How many weeks are in a month?

A.) There are 4 weeks in a month (no more, no less).

Q. What is 7 times 4?

A.) 28.

If there are seven days in a week, and only 4 weeks in a month (no more, no less), then that means a month is only 28 days. However, the Gregorian Calendar shows a total of 12 months, 7 of which have 31 days, 4 with 30, and 1 month of the year, February, uses 28 days except during a leap year. How do we reconcile a 13 month calendar to 365 days in a year?

Q.) If we change all the months of the year to 28 days, what happens to the left over days that gave us 30 or 31 days in a month?

Here’s where the extra month comes in. We’ll do it as a math equation to make it easier…

Remember when I told you that there were 7 months in the Gregorian Calendar that uses 31 days instead of 28? 4 months that use 30? And 1 that uses 28?

IF 31 [days] times 7 [months] = 217 days.

30 days times 4 [months] = 120 days.

28 days times 1 [month] = 28 days

When we add all of the days together, we’ll have a sum of : 365 days!

Now if we divide 365 [days] into 28 [days] the result is 13. 0357214 months in a year (I’ll explain how we lose that .03 margin a bit later).

Not only would we have an equal number of months, but each day of the week will fall on the same day each year, as well as the holidays. The question is why didn’t the Julian or Gregorian Calendar follow the 28 day rule in the first place?

It’s possible that a thirteen month Calendar existed before the change by Julius Ceasar in 46 B.C. For example, the Mayans used a 260 day calendar. If you do the math this equates to a total of ten, 28 day months. But in early Europe, superstition may have played a role in why 13 months were avoided.

If you look at astrology at the time, even the 13th astrological sign had been removed. This sign would have correlated with the missing 13th month.

The astrological zodiac signs begin in March (formerly the first month of the year), with Aries. Because of the omitted 13th month and its correlating sign, Ophiucius… Aries begins on the 21st of March and has a 31 day period just like the month it is supposed to represent. The other astrological signs in our calendar year are the same.

The sun signs represent all of the stars in Earth’s celestial belt. These are constellations that include the sun at some point in its ecliptical field. There are 13 zodiac astrological signs:

Aries, Taurus, Pisces, Leo, Virgo, Scorpius, Saggitarius, Capricorn, Aquarius, Cancer, and Ophiucius.

Ophiuchus is the only one not counted as a zodiac sign, even though it falls in the belt. Ophiuchus is represented by a man with a serpent around his waist. He is thought to represent the Greek God of Health and Medicine: Asclepius.

His image is also thought to represent the image of Adam in battle with the Serpent/Satan. Which could also shed some light on superstitions surrounding this Zodiac sign and why it is not in use. Whatever the reason, at some point in history an extra month was omitted by government and religious leaders and the world forgot.

If you do the math, it adds up.

Q. When I multiply 28 days times 13 months, there are only 364 days.

A.) There’s a mathematical reason why. The reason a year is only 364 days under the 13 month calendar, is because a full day is based on the amount of time it takes Earth to rotate on its axis. However, contrary to what is commonly held belief, a full day is not 24 hours, but 23.934. If we calculate a year based on the exact amount of time it takes the Earth to rotate, we get an approximate answer of 364 days.

The reason we have 365 days under a 12 month calendar is because our clocks are designed to calculate 24 hour days and not its approximate number of 23.934 hours a day. This results in an additional 2 hours a month on our calendar. 2 [hours] times 12 [months]= 24 [hours]. An additional day a year, resulting in 365 days in a 12 month calendar.

I have provided a few math equations to prove this point. I will first calculate a year based on the Earth’s true rotation at 23.934 hours in a day, followed by a calculation based on 24 hours. It looks harder than what’s actually there. But if you read it, you’ll get it right away.

It is likely that in 46 BC when Julius Ceasar made the Julian Calendar, and when Pope Gregory XIII approved the Gregorian Calendar, astrologists were unable to accurately estimate Earth’s rotation at 23.934, and so, opted to round the number to 24, thus making it easier to make a calendar with 12 months, the mishap causing an additional day every four or so years.

The math:

There are 60 minutes in 1 hour.

There are a total of 1380 minutes in 23 hours.

1380 minutes = 23 hours

+ 56 minutes = 1436 minutes in a full day.

1436 x 28 days = 40208 [minutes] = a month

(4.091 [seconds] * 28= 114.548 seconds )

114.548 [seconds] - 60 [seconds] = 54.548 [seconds] =

add the addition 60 seconds as one minute to 40208 + 1 minute = 402099

M = 402099 and 54.548 seconds.

402099 / 28 days = 1436.055

1436.055 / 60 = 23.934.25 [HOURS] in a day

23.934 * 7 = 167.538 [hours] in a week. (OR simply 23.934 * 28 = 670.152 [HOURS])

167.538 * 4 = 670.152 [hours] in a month.

670.152 * 13 = 8711.976 [hours] in a year.

8711.976 / 364 = 23.934 hours = 1 day

based on the accurate 23.934 hours in a day, there are only 364 days in a year.



60 [= 1 minute] * 24 hours = 1440 MINUTES in a day

1440 * 31 = 44640 * 7 [months] = 312480 [minutes per year]

1440 * 30 = 43200 * 4 [months]= 172800 [minutes p.y]

1440 * 28 = 40320 * 1 [month] = 40320 [minutes p.y]

add MINUTES TOTAL IN A YEAR for each = 525600

525600 / 60 [MINUTES] = 8760 HOURS [in a year]

8760 MINUTES / 24 [hours] = 365 DAYS [A YEAR]

(8760 minutes divided by 24 hours calculates 365 days a year).

BECAUSE THERE IS AN EXCESS OF 2 HOURS per month due to rounding off to 24 hours from 23.94, we end up with a leap


2 [hours] * 12 [months] = 24 hours a whole additional day.

From Wikipedia.org - Gregorian Calendar reference:

No. Name Days
1 January 31
2 February 28 or 29
3 March 31
4 April 30
5 May 31
6 June 30
7 July 31
8 August 31
9 September 30
10 October 31
11 November 30
12 December 31


Just FYI, I posted this article on my web site http://theoryuniverse.com - feel free to visit my web site, make an account, and post your own theories! Yes it's not just for science related theories, but social, political, or other theories as well!

- E
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Wednesday, August 13th 2008 at 3:40PM
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lol no it wasnt too boring at all. very insightful!
miss charity
Wednesday, August 13th 2008 at 3:52PM
charity White

Here you go starting trouble again:) The issue of the missing month is not as important as why it's missing! The zodiac, twelve tribes, and some other celestial symbols intentionally leave out the missing tribe/month/ sign. Ophiuchius is not just the missing zodiac sign or month. In astrological science, he is represented as destroying the our known universe: creating a pole shift- - - i.e. the effects of Global Warming.

Some well respected folks even predicted the date that this would happen: 12, 21, 2012.

Wednesday, August 13th 2008 at 3:53PM
Dr. Ahmad Glover
I had this very conversation with my OB/GYN when I was 22 and pregnant with my 1st daughter. I carried her for exactly 40 weeks (she was born the day she was due)...to me, that's 10 months!!!! Man!! What happened to 9???????? I was hot about it. ...turns out a full-term baby is 37-42 weeks from the LMP (last menstrual cycle). Go figure.


Wednesday, August 13th 2008 at 4:13PM
Dee Gray

What is the greco roman or Gregorian Calendar? If we had a perfectly good lunar calendar...why was our current calendar created?
Wednesday, August 13th 2008 at 4:19PM
Dr. Ahmad Glover
E, what happen the zodiac sign, Cancer. You didn't mention it. I am very disappointed that you left it off the list, without an explanation. I can careless about Gemini :) Nah, only kidding, my brother is a Gemini.
Wednesday, August 13th 2008 at 5:47PM
Kenneth X
I have always contested the notion of time. If you are a science nerd (like me) you might find it interesting to know that on the Planck Scale, time doesn't exist:matter just moves at a pace that isn't measurable.
Wednesday, August 13th 2008 at 10:51PM
Steven Jones
Here's the article I was referring to
Friday, August 15th 2008 at 6:41PM
Steven Jones
E Private are you sure that you did not take that PhilosophyMeta physic class on TIME with me? As, I said before this class was one of the most interesting class I have ever taken. I am as math dead now as I was in that class. And you did not use Greek terms which made following what you had to say easier.(smile)the study of time is beautiful and so very breath taking and it sure helps not to be as Number dead as I am....
Thursday, April 10th 2014 at 6:47PM
Kenneth X, I am also a Gemini.EP, do you know any thing about those signs related to the birth month, because I am what my sign says. I am two people just like it says even.
Thursday, April 10th 2014 at 6:47PM
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