«عَبْدِي حُرٌّ إِنْ لَمْ أَضْرِبْكَ حَتَّى يَشْفَعَ فُلَانٌ»
“My slave is free if I do not beat you until so-and-so intercedes.”
Here the action of beating can be continued or repeated, and intervention can be its terminus. The statement concerns an act continuing toward a stated endpoint, rather than merely a single isolated performance.
The same reasoning applies when the event is dawn or nightfall. Stopping before the endpoint differs from stopping when the specified endpoint actually arrives.