Tafsir of Ar-Rum 30:55

{The Hour} The Resurrection. It is named as such because it occurs in the final hour of the world, or because it happens suddenly and unexpectedly—as when you say "in an hour" to someone you are rushing. It has become a proper noun for it, like "the Star" (an-najm) for the Pleiades, or "the Planet" (al-kawkab) for Venus.

{They will swear that they had not stayed [in the world] but an hour} They mean their stay in the world, or in their graves, or in the interval between the end of the world and the Resurrection. In the Hadith: "Between the end of the world and the time of the Resurrection is forty." They said: "We do not know if it is forty years or forty thousand years." That is a time during which they perish and their torment is interrupted. They only estimate the duration of their stay in this manner because they perceive it as short, or because they forget, lie, or guess.

{Thus they were deluded} Meaning: just as they are being turned away from the truth and reality now, they were turned away from it in the world. Their affairs were built upon falsehood. Or, it means: just as they are being deluded now by the realization that their stay was but an hour, they were similarly deluded in the world by their arrogance.

{But those who were given knowledge and faith will say, "You have stayed according to the decree of Allah until the Day of Resurrection; so this is the Day of Resurrection, but you did not know." * So on that Day, their excuse will not benefit those who wronged, nor will they be asked to repent.}