John Calvin Commentary Genesis 18:10

"And he said, I will certainly return unto thee when the season cometh round; and, lo, Sarah thy wife shall have a son. And Sarah heard in the tent door, which was behind him." — Genesis 18:10 (ASV)

I will certainly return to you. Jerome translates it as ‘I will return, life attending me:’ as if God, speaking as humans do, had said, ‘I will return if I live.’

But it would be absurd for God, who here so magnificently proclaims His power, to borrow from humans a way of speaking that would imply He is mortal. What majesty, I ask, would this remarkable oracle possess, which concerns the eternal salvation of the world? That interpretation, therefore, which entirely weakens the force and authority of the promise, cannot be approved.

Literally, it is, ‘according to the time of life.’ Some interpret this as referring to Sarah, as if the angel had said that Sarah would survive to that period. But it is more properly explained as referring to the child, for God promises that He will come at the right and proper time of childbirth, so that Sarah might become the mother of a living child.