Genesis 21:2

For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him.

Sarah conceived, and bore Abraham a son in his old age, at the set time of which God had spoken to him.

And Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him.

and Sarah conceiveth, and beareth a son to Abraham, to his old age, at the appointed time that God hath spoken of with him;

So Sarah conceived and bore a son to Abraham in his old age, at the very time God had promised.