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

1 Answer 1


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

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