Genesis 21:2

At the set time. Like many of God’s promises, this one was fulfilled precisely on time (see chs. 17:21; 18:10, 14). The coming of the Flood, the deliverance from Egypt, the birth of the Messiah, along with various events foretold by Daniel and John, were in fulfillment of time prophecies (Gen. 6:3; Ex. 12:41; Dan. 9:25; Gal. 4:4). Of the repeated promises made to Abraham of the birth of a son, only those immediately prior to that event made specific mention of the time of the event. At first Abraham was informed only that he would have a son. Later he was told that Sarah would give birth to that son, and only at the very last was he told when the son would be born.