Tafsir of An-Nazi'at 79:43

This is a denial and a rebuttal to the questioning of the polytheists regarding it. That is: In what capacity are you to mention its time to them and inform them of it, such that they would ask you to clarify it? This is like His saying, the Exalted: "They ask you as if you are well-informed about it." Thus, the interrogative is for denial.

"In what" (fīma) is a fronted predicate, and "you" (anta) is a delayed subject. "Of its mention" (min dhikrāhā)—upon the estimation of an omitted genitive, meaning: "the mention of its time"—is linked to what the predicate is linked to.

It has been said: "In what" (fīma) is a denial of their questioning, and what follows it is an inception (isti'nāf) acting as the cause for the denial and an exposition of the futility of the questioning; that is: "In what [is the basis for] this questioning?" Then, an inception occurs, and it is said: "You are [the one who is] of its mention," meaning: Your being sent—while you are the Seal of the Prophets, who was sent in the breath of the Hour—is a sign of its signs and a proof that guides them to the knowledge of its imminent occurrence. Thus, this degree of knowledge is sufficient for them.