How can the time of death be determined in a vision, even though no one knows the appointed time of death?

It is not possible to definitively determine one's lifespan through a dream, as dreams may be true or untrue. Moreover, the determination of lifespans is part of the knowledge of the Unseen, which Allah alone possesses. The Messenger of Allah, peace and blessings be upon him, said: "The keys of the Unseen are five," then he recited: "Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted." (Surat Luqman, 31:34) Although a believer's dream may be true when the time draws near, the predominance of truth does not preclude the possibility of falsehood. Furthermore, the interpreter may not be accurate in their interpretation; therefore, dreams should only be interpreted by those who are skilled in it.