وَالْعَصْرِ
By time. (العصر: 1)
The wāw is the particle of oath and al-ʿaṣr is the governed object of that oath. The response follows in the next verse. Such a compact opening creates anticipation: the listener expects a weighty claim after the solemn invocation.
Exegetes discuss the scope of al-ʿaṣr, including time in a broad or more particular sense. The secure rhetorical observation is that time is made salient through an oath. A narrower interpretive option should not be presented as a grammatical necessity of the oath construction.