On the 11th of Cheshvan of the year 1656 from Creation, the soul of Metushelakh left this world. It was exactly seven days before the beginning of the Great Flood.
For in another seven days, I will make it rain upon the earth for forty days and forty nights, and I will blot out all beings that I have made, off the face of the earth.” (Breishit, 7:4)
In his commentary to this verse, Rashi explains:
For in another seven days: (Sanh. 108b) These are the seven days of mourning for the righteous Metushelakh, for the Holy One, blessed be He, had consideration for his honor, and delayed the retribution. Go and calculate the years of Methuselah and you will find that they ended in the six-hundredth year of Noah’s life.
For in another seven days: What is the meaning of “another” ? A [period of] time following a [period of] time; this [period] was added to the one hundred and twenty years. — [See Rashi 6:14]
Noah, the only person who, together with his family, survived the global cataclysm, was the grandson of Metushelakh.