Charles Ellicott 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)

According to the time of life. —Hebrew, according to the living time. It is evident from Genesis 18:14, and 2 Kings 4:16–17, that these words denote some fixed period, but the exact rendering is in dispute. The interpretation “When the season revives” (meaning next spring) is entirely remote from Eastern thought, and the rendering of Zunz, “at the living time,” is poetical but meaningless.

The true rendering is probably “a year from now,” as when the year is over it dies, and a new year lives in its place. Jewish tradition is strongly in favor of this view, translating it as “according to this time next year” and adding that the season was the Passover. The only other tenable rendering is “in course of time.”

Which was behind him. —The Septuagint has a preferable reading, and she was behind it. The door, as we have seen, was an opening made by looping back the curtain, which would effectively conceal Sarah’s person.