Did Noah really take 120 years to build the ark?
First Key : The year of Shem's Birth (Noah 502 yrs. old)
In order to calculate the time it took Noah to build the ark, we must first know the year of Shem's birth because God commanded Noah to build the ark after his three sons had been born (Gen 6:10~). Genesis 5:32 testifies, "Noah was five hundred years old, and Noah became the father of Shem, Ham, and Japheth." This verse simply informs us that Noah had three sons after he turned 500 years old. In order to know the exact year of Shem's need to read
another verse. Genesis 11:10 states, "These are the records of the generations of Shem. Shem was one hundred years old and became the father of Arpachshad two years after the flood." Through these two verses, we can figure out the exact year of Shem's birth. Since the flood took place when Noah was 600 years old, Shem turned 100 in the year Noah turned 602. Therefore, Shem was born when Noah was 502 years old which was 98 years before the flood. Just knowing Shem's year of birth shows us that the duration of building the ark was less than 100 years.
Second key : Ham and Japheth had been born when the command to build the ark was given.
The point in time when God revealed His plans about the flood to Noah (Gen 6:13) and when He gave the command to build the ark along with the precise plans for the ark are very important. Genesis 6:10 tells us that God commanded Noah to build the ark after he had already given birth to his three sons, Shem, Ham and Japheth. Since Shem was born when Noah was 502 years old, it would've taken Noah at least 2 more years to give birth to both Ham and Japheth (since they were't twins) and 4 or more years if they were birthed 2 or more years apart. Therefore, since the flood took place when Noah was 600 years old, the time it took to build the ark must've been fewer than 96 years.
Third key : Noah had daughters-in-law when he received the command to build the ark.
When God commanded Noah to build the ark in Genesis 6:18, He said, "but I will establish My covenant with you; and you shall enter the ark - you and your sons and your wife, and your sons' wives with you." In other words, when Noah received the command to build the ark. his sons, who had been born to him after his 500th year, had already grown and gotten married. For his three sons to have been married, they must've been at least 15 years old. Considering such factors, Noah and his 7 family members (Noah's wife, three sons, and three daughters-in-law) could not have built the ark for more than 70 to 80 years.
Fourth key : The day the ark got completed (Noah's 600th year)
The flood starts on the 17th day of the second month of Noah's 600th year (Gen 7:11). Noah and his family receive the revelation about the flood judgment along with the command to enter the ark 7 days before the flood (Gen 7:1-4). If the ark had not been finished according to the plans that God had given, then it would not have survived the flood. Therefore, the ark must've been completed before the 10th day of the second month of Noah's 600th year.
Fifth key : 120 years is not how long it took to build the ark.
Let us carefully reread Genesis chapter 6 as it is recorded. It has been proposed by many commentaries and expositions of Genesis that the passage "his days shall be one hundred and twenty years" in Genesis 6:3 is a reference to the time it took to build the ark. The 120 years here alludes to the time when God warned about the impending judgment to a fallen, carnal-minded world. God did not give the command to build the ark at that time. If the ark took 120 years to build, then Noah's age 120 years before the flood would be 480. Then that would be 22 years before his firstborn, shem, was born. Such facts would go against the testimony of the Bible.
The flood came the year Methuselah died at the age of 969
BIRTH OF METHUSELAH (THE OPENING ACT FOR JUDGMENT OF THE FLOOD)
At the sound of the baby's cry, Enoch watched the lovely newborn as he felt mixed emotion of anxiety and attachment. He was 65 years old. He named his son, "Methuselah."
Methuselah was his name. Why would he give his beloved son such a startling name?
Near the birth of Methuselah, Enoch received a special revelation from God. It was the revelation about the judgment of the Flood which would bring this world to its end. That revelation seemed to include the fate of his soon-to-be born son. What a terrifying revelation was this: "When he dies, the judgment will come." The moment Enoch gives this unwilling name unto his child, he faced a great turning point in his life. Ever since then, his life was to walk with God (in Hebrew text, it means "subject one's will to God and follow Him"). Every time he called his son by this terrifying name, he was reminded of the startling message for the judgment (Jude 1:14-15).
METHUSELAH AT AGE 187: BIRTH OF HIS SON, LAMECH
Methuselah grew up being called by others "end of the world." He watched his father Enoch's godly life, and when he became 187 years old, he had a son, Lamech. Enoch his father was 252 years old. These three generations-Enoch, Methuselah and Lamech - builds the household of faith upon the teachings of godly Enoch.
METHUSELAH AT THE AGE OF 300: TRANGFIGURATION OF ENOCH HIS FATHER
In the 365th year of Enoch who had been walking with God, he did not see death and was taken up to heaven. Adam demonstrated that the sinful mankind cannot evade its fate of death. Enoch himself showed that if one walks with God, he will not see death. Since Methuselah was left alone to witness this crossroads of fate, he must have reconsidered the meaning of his name given by his father.
METHUSELAH AT AGE 369: BIRTH OF NOAH HIS GRANDSON
Now, Methuselah also became a grandfather. His son Lamech became father to Noah. The name "Noah" means "comforter, who gives peace." This name bore Lamech's agonized appeal upon realizing the frailty of man.
METHUSELAH AT AGE 800~900: NOAH RECEIVES THE COMMAND TO BUILD AN ARK
Much time passed. His 7th-generation ancestor Seth, 6th-generation ancestor Enosh, 5th-generation ancestor Kenan, Great-grandfather Mahalalel, and Grandfather Jared all passed away in that order. From his grandson Noah, three sons were born - Shem, Ham and Japheth.
His great-grandfather lived to the age of 895 and his grandfather Jared lived to the age of 962. It was now the turn of Methuselah, who was almost 900 years old, to settle for his life. But, he could not pass away in peace like his forefathers. How would his name given by his father Enoch be fulfilled? How would the fulfillment of God's revelation that had governed his entire life to this day come? As the time passed and wickedness filled the earth, Methuselah had to live his days in fear and suspense... Then, God commanded his grandson Noah to build an ark.
THE LAST 70-80 YEARS OF METHUSELAH
The family of Noah his grandson built the ark through ridicule and scorn. Even his close relatives despised and avoided Noah and left him in utmost solitude. Noah's grandfather, Methuselah, saw this and must have helped Noah in building the ark. He must have encouraged and strengthened Noah. His heart must also have been filled with frustration and longing toward the world which refused to listen to Noah's cry.
Methuselah passed the age of 962, Which was the age his Grandfather Jared Passed away as the longest living person. When he was 964 years old, his son Lamech also passed away before him. Methuselah had to live longer than his own son. He was aware of the significance of his unprecedented longevity. "When you die, the end will come..." Each day as he watched the sunrise, he tearfully feels God's mercy and compassion in withholding the judgment to wait for sinful mankind to repent.
METHUSELAH AT AGE 969: AND...
His grandson Noah was 600 years old and his great-grandson Shem is 98 years old.
He had been called throughout his life by the name, "When he dies the end will come." Methuselah who witnessed the death of the "first man" Adam and the transfiguration of his father Enoch, breathed his last breath at last. And, in the 17th day of the 2nd month in the same year, the flood came and the ungodly people's world perished.
This is a partial summary of excerpts from The Genesis Genealogies