How Long is a Month in the Bible?

Bible months are a predictable 30 days each. No exceptions. There is no attempt to make the month synchronous with the movements of the earth’s moon. It also makes time counted with months quite precise.


How long is a month?

Since there is no place in the Bible where the shape of the calendar is spelled out completely, the answer must be found by scavenging references to time given across the Bible. At least 4 separate stories indicate a month as referenced in the Bible is 30 days long.


The first place to find enough references to compute the length of a month as used in the Bible is found in Genesis chapter 7 and the account of Noah’s flood. The opening of the flood, the day Noah and his family entered the ark, is clearly documented.

11In the 600th year of Noah's life in the 2nd month in the 17th day of the month, on that very day all the fountains of the great deep burst forth and the windows of the skies were opened. 12And the rain fell on the land for 40 days and 40 nights.

13On that same day Noah and Shem and Ham and Japheth, the sons of Noah, and Noah's wife, and the 3 wives of his sons with him, entered into the ark; Genesis 7:11-13

Waters began to flood the earth from the sky, and waters came up from the "great deep" covering the planet. On this same day Noah and his family entered the ark and Yahvah sealed them in.Genesis 7:16

The story continues with details regarding a the first 40 days of the flood which were more intense than the remaining 110. The story also recounts that everything in the waters died; everything with the breath of life. The account of Noah’s flood finally concludes with a restatement that the waters prevailed upon the earth for 150 days.Genesis 7:24

Then the story turns to the end of the flood and the ark coming to rest on the mount of Ararat. Genesis 8:3-4

Two important details spring from this story:

  • The date was month 7 day 17.

  • After the 150 days of flooding.

The detail that this day, the day when the ark landed, was after the 150 days of flooding matters to the precision of this story.

Recall the principle of counting in the Bible. Any fraction counts towards a whole. The first day of the 150 was the day Noah and his kin entered the ark. This is the day the rains began. Though it wasn’t raining when the day began, this is day 1 in the 150 days being used to count out the days of rain in this story because sometime on this day the rains started. Rains on this day cause it to be counted as day 1.

Day 150 in this story is month 7 day 16 exactly the day before the date given in the passage above. Remember, it was after day 150 of rain that the ark rested on Ararat.

Now, what we’re trying to understand is the number of days in the months used to record this story. Instead of using an algebra formula, in this case a simpler answer is to use a table. The left hand side of the table is the day number, the right hand side is the date. At this point three rows are known:

Day Number Noah’s Date
1 -- Rains begin 600-02-17 -- Noah enters ark
... ...
150 -- Floods abate 600-07-16
151 600-07-17 -- Ark rests

The dates given in this table are of the form year/month/day. The day field is what drives the number of days in the month. To fill out the table, supplying the missing 148 rows, requires an answer to the question. How many days are there in each month?

It should be obvious that the answer is roughly 150 days / 5 months = 30 days/month.

If this is the right answer the table can be filled out with 30 days in each of the months going down the table. In order to save space the following is the same table as above, skipping some of the rows that don’t add much to this story, but filling in all the rows were things interesting happen.

Day Number Noah’s Date
1 -- Rains begin 600-02-17 -- Noah enters ark
2 600-02-18
3 600-02-19
... ...
14 600-02-30
15 600-03-01
... ...
40 -- Rains end 600-03-26
... ...
44 600-03-30
45 600-04-01
... ...
74 600-04-30
75 600-05-01
... ...
104 600-05-30
105 600-06-01
... ...
134 600-06-30
135 600-07-01
... ...
150 -- Floods abate 600-07-16
151 600-07-17 -- Ark rests

Error Analysis

This is a perfect fit, with no errors across the 150 days. The above table works exactly.

Degenerate Modern Case

To understand more completely what is going on here, consider if Noah’s flood happened at some point in the modern era. Let’s just say, for the sake of argument, that Noah’s flood happened on February 17, of 2029.

The year 2029 is not a leap year, and is structured like all other non-leap years in our era. The following example works for any modern year provided it is not a leap year. It would work the same for 2007, 2009 and 2010.

The following table is like the previous table but now the dates are modern dates from the modern Gregorian Calendar. Note carefully how the same day 150 flood abatement lands on the same month and day as given in the account of Noah’s flood.

The following table begins and ends the same way as it did in the original account. The interior is subtly different. Think about why this is so.

Day Number Noah’s Date
1 -- Rains begin 2029-02-17 NS -- Noah enters ark
2 2029-02-18 NS
3 2029-02-19 NS
... ...
12 2029-02-28 NS
13 2029-03-01 NS
... ...
40 -- Rains end 2029-03-28 NS
41 2029-03-29 NS
42 2029-03-30 NS
43 2029-04-01 NS
... ...
73 2029-04-30 NS
74 2029-05-01 NS
... ...
104 2029-05-31 NS
105 2029-06-01 NS
... ...
134 2029-06-30 NS
135 2029-07-01 NS
... ...
150 -- Floods abate 2029-07-16 NS
151 2029-07-17 -- Ark rests

So why does this work?

This works because February is only 28 days long, 2 days shorter than the average. These 2 missing days in February are made up for in March and May, both of which have 31 days. The result exactly averages to 30 days/month.

This works for all non-leap years in the modern era using the modern calendar.

Was the modern Gregorian calendar, or for that matter the Julian Calendar, used to write the Bible? Used to write the account of Noah’s flood? No.

The written account of Noah is likely 3500 years old or more. This precedes the introduction of this calendar by about 1500 years.

No known ancient calendars had an upper length for months at 31 days while at the same time kept a short leap month at 28 days. This is the feature needed to make this work.

It should also be noted, the feature of the modern calendar that makes this work is the short 28 day February. Across most of the era since the time of Jesus, March 25 was New Years day. This makes month 2 out to be April, and this degenerate case would not have worked in any of those years.

Days/Month from Revelation

The number of days per month of time can also be independently established from at least one reference from the Book of Revelation.

1And there was given to me a reed like a rod; and the king stood saying, Arise and anoint the temple of god and the altar and those who worship in there. 2But leave out the outer court of the temple, and do not anoint it; for it has been given to the peoples; and they will tread the holy city under foot for 42 months. 3Then I will give power to my 2 witnesses, and they will prophesy 1,260 days, clothed in sackcloth. Revelation 11:1-3

To be able to use this reference as a basis for computing the number of days in a month the equivalence of these two intervals must be shown. Understand carefully this problem. Are the 42 months the same time interval as the 1260 days?

If they are not the same interval, then the number of days in the 42 months may or may not be 1260 days and the number of days per month cannot be computed from this reference.

Reading the text of the verses surrounding these two time references does not help. The 42 months are a time when the holy city shall be tread under foot while the 1260 days are when two witnesses shall prophesy and be dressed in sackcloth. Are these the same thing or are they different?

To make matters worse the number of days/month has already been shown to be 30 days/month so the math is screaming out that these are the same. Why? Because 42 months * 30 days/month = 1260 days.

But, just because the math works does not prove that these two intervals are the same.

There is a principle in interpreting the Bible that helps in this situation: Every matter is to be established by the testimony of two or more witnesses. Deuteronomy 17:6 Matthew 18:16 Second Corinthians 13:1

This same principle was also clearly used in the writing of the Bible to begin with.

Under this principle every conclusion that might be brought from any one passage of the Bible must be established from some other passage. Since it was written with duplication in mind we can even go so far as to state that until the second references are known no conclusions can be drawn from the Bible.

This is a powerful foundation. It also means that references like 42 months must be stated somewhere else, at least 1 time.

Without giving a full defense here, let me state that time references, especially the prophetic end-times references, are given with different units. This reference to 42 months must be given as a day count, or as a year count, somewhere else, using different units.

If the 42 months, is not the 1260 days then there must be a reference to some other number of days that would be the day count instead. A lunar month is about 29.5 days and a solar month is about 30.5 days so the range of possible day counts that would be an independent statement of 42 months would be between about 1240 days and 1280 days. No such day count references exist in the Bible.

Curiously several other references that confirm the 1260 day counts for 42 months are found in the very next chapter of Revelation. Revelation 12:6Revelation 12:14 Here the prophetic expression "time" 2 times and 1/2 time" are equated again to 1260 days. Since "time" in one of its prophetic uses means years, this means the 12 + 24 + 6 = 42 months in 3 and 1/2 years.

Bottom line here is the math works to confirm what Noah’s Flood first showed:

1260 days / 42 months = 30 days/month


Daniel is also shown about the events leading up to the end of the age. In addition to being told his book is sealed until the time of the end of the age, he is also given nearly the same mathematics as John was given in Revelation, except the stated day count is 1290 days.

5Then I Daniel looked, and saw 2 others; one stood on this side of the bank of the river and the other on that side. 6They said to the man clothed in linen who stood above the waters of the river, How long will it be until the end of these things? 7I heard the man clothed in linen who stood above the waters of the river. He held up his right hand and his left hand to the skies and swore by him who lives forever that, It will be for a time, times, and half a time, and when the holy people are delivered all these things will be fulfilled.

8I heard something but I did not understand. Then I said, My master what will come after these things? 9He said, Go your way Daniel, for the words are closed up and sealed until the time of the end. 10Many will be chosen and made white and tried, but the sinners will continue to sin and none of the wicked will understand, but those who have done good works will understand. 11From the time that the daily sacrifice will be abolished and the abomination is given to destruction there will be 1,290 days. Daniel 12:5-11

Unlike John’s account in Revelation, we can show that the 3.5 timesDaniel 12:7 is the same as the number of days. Why? Because Daniel did not understand the 3.5 times reference so he is told again this time a count of 1290 days.

This is very much like Revelation except the number of days is not the same. Importantly the number of days given to Daniel compared to the number of days given to John is exactly 30 days higher.

Why would this be so?

Since the 1290 days is linked to 3.5 years this count of days must still be counting exactly the number of days in some set of months. So the 30 day difference is a difference of 1 month, confirming the count of 30 days/month.


The Book of Zechariah has an obscure reference that also confirms the number of days in a month. The passage is in a prophecy given about Jesus. It will be fulfilled when Judas betrays Jesus for 30 pieces of silver paid him by the Sanhedrin in Jerusalem. The following is part of the original account.

83 shepherds I did away with in 1 month; and my soul was wearied of them, and their soul also howled against me.

9Then I said, I will not feed you; what dies, let it die; and what is to be lost, let it be lost; and let each of the rest eat the flesh of another. 10And I took my staff, even Pleasant, and I broke it, thus nullifying the contract which I had with all the peoples. 11And it was nullified in that very day; and so the meek of the flock who were watching me, knew that it was the word of Yahvah.

12And I said to them, If it seems good in your sight give me my wages, and if not then you are doing me injustice. So they weighed for my wages 30 pieces of silver. Zechariah 11:8-12

The reference to 1 month in this passage is repeated again using different units at the end of this passage when the price to betray Jesus is stated for the first time as 30 pieces of silver.

Why would month be linked to 30 pieces of silver?

A general prophetic principle is that all counts of anything, including units of currency, are also units of time. Genesis 40:12Genesis 40:18 The price is 1 silver coin for each day in the month when Jesus cuts off the three shepherds.

This is a veiled prophetic reference that indicates the time of Jesus’ earthly life will be 1 month. Using the standard day-for-a-year ratio where a day as spoken to the prophet is a year in the history of the nation Jesus’ ministry on earth is being predicted at 30 years.

This will be born out when we get to the topic of Jesus’ life and ministry. Jesus lived 40 days longer than 30 years. It was exactly 30 years between his day 40 appearance at the temple as an infant and his crucifixion.

Jesus’ life followed the standard day-for-a-year ratio, but in this case, it was a reverse. 1 year for each of the 30 days in a month.

It appears that the price for a slave is 1 coin per year of age. Joseph was apparently priced this way. This is why the price is mocked in this passage and why Judas behaves by throwing it back into the temple.

Disproving Roman and Jewish Calendars

Revelation asserts that across long time spans that the month length averages to 30 days. Neither the Roman nor the Jewish calendars exhibit this quality the Roman with a slightly longer month length, the Jewish with a slightly shorter month length.

It is easy to show that neither the Roman Solar nor Jewish Lunar-Solar calendars were used in the Bible.

The average Roman month length is 30.44 days/month. This is approximate, derived from 365.25 days/year / 12 months/year. If the Bible was using the Roman calendar the period 42 months would take over 1278 days, 18 days longer than given in Revelation.

The Roman calendar and any related or similar calendar cannot be the calendar used to write Revelation.

If the Bible was using the Jewish Lunar calendar with an average month length of 29.45 days/month. This is also approximate. 42 months would be under 1237 days, 23 days shorter than given in Revelation.

The Jewish calendar and any related or similar calendar cannot be the calendar used to write Revelation.


Though the modern calendar can be shown to map to Noah’s flood in various degenerate ways, it is not possible for the modern calendar to have been used for the writing of Revelation, nor the rest of the Bible.

The calendar used to write the Bible has a strict 30 days/month as seen in Revelation, Daniel, and Genesis. This same 30 day/month value is used prophetically in places like Zechariah.


Knowing all months are of equal length greatly simplifies the process of reconstructing the time line of the Bible. No astronomical calculations are needed, the calendar is not tied to the movements of the sun nor moon nor starry host. Not much more than a high school programming class and a Bible are needed to unravel this riddle.

As will be seen, it is possible to even day-accurately place the timing of Noah’s flood onto the modern calendar. Read on for more.

Phil Stone, Updated 2016-11-04