I propose to you that God, too, works against deadlines that He sets. Here in Genesis 17 and in verse 20 it says:
There is a deadline—"at this set time." Now maybe Abraham and Sarah did not know exactly when that would be, but God did. Maybe He did reveal it to them, but I do not think He did. But it was set in God's mind when that time would be.
What God did here was add circumcision as a requirement and as the sign that those bearing this mark had made the covenant at the same time with Abraham. (I believe that this term "selfsame day" appears something like fourteen times in the Bible.)
