They entered on the 17th day of the second month and the waters prevailed 150 days until the 17th day of the 7th month; thus making 5 months equal to 150 days therefore each month 30 days in length*.
Here is the chronology of the Flood taken from Genesis 7–8.
|Enter the Ark||2||17||0||1|
|Rain ceases (inclusive)||3||26||40|
|Rain ceases (exclusive)||3||27||40|
|Waters dried up||1||1||314||315|
|Earth dry, leave Ark||2||27||370||371|
It seems that the Bible more frequently uses inclusive reckoning, either way they spend 371 days inside the Ark. While I have known of this number for some years, I noticed it coincided with the duration of a solar year based on a 30-day month with an altered earth spin as per my previous calculations. This seems a little more than coincidental. If this were the case perhaps the Noadhic calendar was a lunar-solar calendar similar to the Hebrew calendar and various other calendars.
A lunar-solar calendar aligns the months with the moon (new moon to new moon) and has a variable number a months to keep the seasons aligned. That is 12 months each year with an extra month approximately every 3 years (both antediluvian and postdiluvian solar years have a surplus of ~11 days over the lunar year).
The antediluvian lunar-solar calendar would have 12 30-day months for a 360-day year. The postdiluvian lunar-solar calendar has ~29.5-day months, thus alternating between 29- and 30-day months, but depending on when the new moon appears, for a 354-day year.
*This seems to be the most likely interpretation but the weather may have precluded accurate visualisation of the moon and 30-day months may have been used until adequate moon sighting allowed resetting of the calendar. The second option raises the question as to why not use alternating 29 and 30 day months until the calendar could be corrected.