John Calvin Commentary Genesis 27:2

"And he said, Behold now, I am old, I know not the day of my death." — Genesis 27:2 (ASV)

Behold, now I am old, I know not the day of my death. There is not the least doubt that Isaac implored daily blessings on his sons all his life: this, therefore, appears to have been an extraordinary kind of benediction. Moreover, his declaration that he did not know the day of his death means that death was pressing so closely upon him—a decrepit and failing man—that he did not dare promise himself any longer life.

Just as a pregnant woman, when the time of childbirth draws near, might say that she had no certain day left. Everyone, even in the prime of life, carries a thousand deaths with them. Death claims the fetus in the mother’s womb as its own and accompanies it through every stage of life.

But as death presses the old more closely, so they should keep it more constantly before their eyes and should pass through the world as pilgrims, or as those who already have one foot in the grave. In short, Isaac, as one near death, wishes to ensure the Church's survival through his son.