Tafsir of An-Naba' 78:17

"Indeed, the Day of Judgement was a fixed time"—this is a commencement to clarify the reason for delaying that which they ask about and hasten, saying: "When will this promise be, if you are truthful?" It serves as a type of detail for the manner in which it will occur, and what they will encounter at that time of various forms of torment, according to what the general warning previously established.

Some eminent scholars said that since He (Glorified be He) established the validity of resurrection, it became a point of inquiry regarding its time. Thus, it was said: "Indeed...", and it was emphasized because it is something they doubted, though it is not as they imagine. That is, the Day on which Allah (Exalted be His status) separates the creatures was, in His (Mighty and Majestic is He) knowledge, a fixed time and a promised appointment for the resurrection of the first and the last, and for the requital that follows it, in terms of reward and punishment; it is something that can hardly be bypassed by being early or late.

It has been said that it is a limit by which the world is measured and at which it ends, or a limit for the creatures to which they will arrive for the distinguishing of their conditions. The first is more fitting for the context, given that the world ends, according to some, at the first blast of the Horn. Regardless, the use of the past tense in "was" (kana) is in consideration of His knowledge. It is also permitted that it carries the meaning of "will be" (yakunu), where the past tense is used for the future to indicate the certainty of its occurrence.