Young's Literal Translation

Genesis 7:11


In the six hundredth year of the life of Noah, in the second month, in the seventeenth day of the month, in this day have been broken up all fountains of the great deep, and the net-work of the heavens hath been opened,


Genesis 7:24


and the waters are mighty on the earth a hundred and fifty days.

Genesis 8:3-5


And turn back do the waters from off the earth, going on and returning; and the waters are lacking at the end of a hundred and fifty days.

And the ark resteth, in the seventh month, in the seventeenth day of the month, on mountains of Ararat;

and the waters have been going and becoming lacking till the tenth month; in the tenth month, on the first of the month, appeared the heads of the mountains.


These passages explicitly say the waters were strong for 150 days, and then marks out 5 month difference from the 17th of the second month to the 17th of the seventh month. Everything I have ever read said a month followed the phases of the moon (new moon meant a new month), which would have varied from 29 to 30 days, especially in a 5 month period. Even the modern Hebrew calendar varies the length of a month from 29 to 30 days.

Does this passage imply a month is reckoned as 30 days long in the Bible (at least in the Old Testament)?

  • There are two famous ancient civilizations which deeply influenced Biblical Judaism: Egyptians and Babylonians. The latter are known to possess a Flood narrative, entitled Gilgamesh. The former have their entire calendar organized around the regular flooding of the Nile. Furthermore, both their calendars are organized into 30-day months, with the mention that Egyptians added a 5-day period at the end of each group of 12 months, whereas the latter added an extra 30-day month each 6 years. On average, both had a mean year of 365 days, about ten days longer than a lunar year (Genesis 7:11, 8:14).
    – Lucian
    Sep 25, 2018 at 1:00
  • I would be curious to know what the numbers look like in other sources, e.g. the Samaritan bible, Septuagint, Vulgate, etc. Maybe there is some variation there. Sep 26, 2018 at 8:49
  • @TimBiegeleisen: Samaritan Pentateuch, Septuagint, Vulgate.
    – Lucian
    Sep 27, 2018 at 6:46

3 Answers 3


The Jewish Encyclopedia points to that very math in the same passage to deduce that Moses was assuming a 30 day month. However, 30 does not divide evenly into 365 days so it was necessary for the Jewish calendars to be adjusted periodically.

Here is the first part of the entry under the "Month" entry:

A unit of time; the period between one new moon and another. According to the account of Creation in Genesis, it was decreed that the "lesser light" should "rule the night" and serve "for signs and for seasons" (Gen. i. 14). The Psalmist also says, "He appointed the moon for seasons" (Ps. civ. 19). In round numbers thirty days constituted a month, as is evident from the Flood narrative, counting 150 days for five months from the 17th of the second to the 17th of the seventh month (Gen. vii. 11, 24; viii. 4); and the mourning period, reckoned as a full month in Deut. xxi. 13, is elsewhere fixed at thirty days. That twelve months constituted a year also is evident from the Flood narrative (Gen. viii. 5-13)...


  • 1
    That is a great reference. It also says a month is the period from one new moon to the next too, which is closer to twenty nine and a half days. I am trying to answer another question and want to know if when counting months if I can just say each one is 30 days.
    – colboynik
    Sep 24, 2018 at 22:11
  • Well, it appears that to do so is not without good precedent. And yes, I love that Encyclopedia!
    – Ruminator
    Sep 24, 2018 at 22:26

In the beginning, according to Genesis 7:11, the days of Noah were marked by extraordinary events. The Flood came upon the earth on the seventeenth day of the second month, and the fountains of the great deep burst forth while the windows of heaven were opened. The deluge lasted for forty days and forty nights, as mentioned in Genesis 7:24, bringing great upheaval to the world. The waters prevailed upon the earth for a hundred and fifty days.

5 months for a hundred and fifty days gives is 30 days months and 360 days a year.

Interestingly, during the time of King Hezekiah, as recorded in 2 Kings 20:11, a miraculous event took place. Prophet Isaiah prayed to the Lord, and He caused the shadow on the sundial of Ahaz to go ten degrees backward. In this ancient timekeeping system, the clock encompassed 360 degrees, and each hour was equivalent to 30 degrees. Therefore, ten degrees represented a time shift of approximately 20 minutes. This extraordinary occurrence highlights the power and sovereignty of God over time and the natural order.

Based on this understanding of time and its significance, we can explore an intriguing notion. If we calculate 360 degrees multiplied by 20 minutes and then divided by 60 minutes per hour and 24 hours per day, we get 5 days. This is why we have 365 days in our modern calendar. It appears that God, in His divine wisdom, accounted for this time discrepancy to align the natural world with the cycles He ordained.

Looking ahead to the words of Jesus in Matthew, where He spoke of the end days being like the days of Noah, we may contemplate the potential return to a 360-day calendar. Just as the days of Noah were marked by unique events and divine intervention, the end days could witness a similar transformation of time. However, it is crucial to remember that these are speculations, and the workings of God's plans are beyond our comprehension.

  • 1
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    Jul 18, 2023 at 17:31

There is no evidence that Genesis 7 and 8 related to the claim in OP's question.

Logically, the narrative that written at any time should be understandable by the audience at that time. Though Moses was inspired by God to write the Genesis, it was still likely the calendar used was Egyptian based, for they were expatriate from that place.

It may be worth to note that the scripture favor to use day as the unit, when it describes exact period. For example;

  • 'For forty days the flood kept coming on the earth' Gen 7:17 NIV
  • The waters flooded the earth for a hundred and fifty days. Gen 7:24 NIV
  • taking a full forty days, for that was the time required for embalming. And the Egyptians mourned for him seventy days. Gen 50:3 NIV
  • Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to the Lord. Lev 23:16 NIV
  • And I will appoint my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth. Rev 11:3 NIV

The Israel calendar was established under the instruction of God from Exodus 12:2. The Lord did not specific how many days a month, that implied the reform only referred to the beginning of the year cycle, but no change in days.

1 The Lord said to Moses and Aaron in Egypt, 2 “This month is to be for you the first month, the first month of your year.

The ancient Egyptian calendar is said to have three seasons, each of which consisted of four months of 30 days each. So it might be more appropriate to say a month of 30 days long is not originated from Genesis 7 and 8, but Genesis 7 and 8 adopted the ancient Egyptian calendar in its narrative.



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