The Genesis account of Noah's Flood gives dates for some events and timelines for others. If you take the time to reconcile all of these figures, you can learn some important things about how the Bible is structured and how it treats the passage of time.
| Event | Reference | Text Date/Time Lapse | Calendar Date |
| Enters Ark | Genesis 7:1-10 | 10th of Second | 10 Cheshvan or 10 Iyyar |
| Rains begin | Genesis 7:11 | 17th of Second | 17 Cheshvan or 17 Iyyar |
| Rains stop | Genesis 7:17 | 40 days | ~29 Kislev or 29 Sivan |
| Waters began to abate | Genesis 7:24, 8:03 | 150 days | ~17 Nisan or 17 Tishrei |
| Ark on Ararat | Genesis 8:04 | 17th of Seventh | 17 Nisan or 17 Tishrei |
| Mountain tops seen | Genesis 8:05 | 1st of Tenth | 1 Tammuz or 1 Tevet |
| Raven sent | Genesis 8:6-7 | 40 days | ~10 Av or 10 Shevat |
| Dove sent/returns | Genesis 8:8-9 | 0 days | |
| Dove sent/leaf | Genesis 8:10-11 | 7 days | ~17 Av or 17 Shevat |
| Dove sent/no return | Genesis 8:12 | 7 days | ~24 Av or 24 Shevat |
| Waters dried | Genesis 8:13 | 1st of First | 1 Tishrei or 1 Nisan |
| Earth dried | Genesis 8:14 | 27th of Second | 27 Cheshvan or 27 Iyyar |
Some observations from this data:
A lunar month is 28-29 days, depending on the relative positions of the earth, the moon, and the sun. This means that the only way for the dates in the text to work is if the 40 days of rain in 7:17 are included in the 150 days of the water prevailing in 7:24, ending in the last half of the month of Nisan (or Tishrei if the "first month" is meant to refer to Nisan instead of Tishrei).
The dates of some events have to be approximated for two reasons.
First, since a lunar month is only 28-29 days, it is impossible to get 150 days between the 17th day of the Second month (Cheshvan) and the 17th day of the Seventh month (Nisan). That would be 145-147 days. Then from the 17th of Nisan to the 1st of Tishrei is actually about 157-160 days. The two periods combined are close to the specified 300 days, but the text actually says that each period was 150 days. Clearly the author was rounding the numbers to a neat 150 and didn't mean for them to be understood as mathematically exact.
Second, counts of days are frequently rounded in Scripture. "Forty days" is rarely meant to be a literal and exact 40 days, but some period of time greater than one month and less than two. In English today we might say "several weeks" or "a month and a half" to mean the same thing, when we almost never have an exact count of days in mind.
Whether the flood began in the month of Cheshvan or in the month of Iyyar depends on what is meant by the "first month", "seventh month", etc. When Genesis was first written, the Jewish people hadn't yet adopted the Babylonian names for the months, so they were only numbered. The Babylonians and Israelites both had two primary new years days, 1 Nisan and 1 Tishrei, for counting different annual cycles. However, in Exodus 12:2, God told Israel that they must begin counting Nisan as the first month of the year. This implies that they weren't doing so before and 1 Tishrei had been the primary new years day up until the Exodus from Egypt. Therefore, I prefer the traditional reconning of the start and end of the Flood in the Fall rather than the Spring.
The waters drying from the face of the earth on the first day of Tishrei, also known as the Feast of Trumpets or Yom Teruah, is appropriate, because that day announces the coming day of liberation and restoration on Yom Kippur, the tenth day of Tishrei.
