Originally Posted by Who am I

Albert Barnes has a good explanation, so I’ll just post what he says.

Noah delays apparently another month, and, on the first day of the new year, ventures to remove the covering of the ark and look around. The date of the complete drying of the land is then given. The interval from the entrance to the exit consists of the following periods:


Rain continued--------------------------------------40 days
Waters prevailed------------------------------------150 days
Waters subside-------------------------------------99 days
Noah delays-----------------------------------------40 days
Sending of the raven and the dove-----------20 days
Another month--------------------------------------29 days
Interval until the 27th of the 2nd month------57 days
Sum-total of days-----------------------------------365 days


Hence, it appears that the interval was a lunar year of three hundred and fifty-six days nearly, and ten days; that is, as nearly as possible, a solar year. This passage is important on account of the divisions of time which it brings out at this early epoch. The week of seven days is plainly intimated. The lunar month and year are evidently known. It is remarkable that the ten additional days bring up the lunar year in whole numbers to the solar. It seems a tacit agreement with the real order of nature. According to the Hebrew text, the deluge commenced in the 1656th year of the race of man. According to all texts it occurred in the time of Noah, the ninth in descent from Adam.