And when the messengers are appointed a time

Meaning: when they reach their appointed time which they were awaiting, which is the Day of Resurrection.

It has been permitted that the meaning is: a time has been determined for them at which they shall appear to testify against the nations upon its arrival and occurrence. The former view is the primary one, as Jarallah stated. Its verification, as found in al-Kashshaf, is that the tawqit (timing) of a thing is the specification and determination of its time. Its application to the individuals [the messengers] requires an implicit ellipsis (idmar), because what is timed is events, not physical bodies. It also carries the meaning of making a thing reach its determined time; upon this, it applies to them without an ellipsis, provided there is a connection between them and that time.

The reason it is the primary view is that the Resurrection is a time in which the time of the messengers—who appear to testify—is clarified; indeed, it is that very time itself. "And when the messengers are appointed a time" necessitates this, for when you say, "When you honor me, I will honor you," it necessitates that the time of the addressee honoring the speaker is what "when" (idha) indicates, whether you make the adverb governed by the verb or by the requital (the main clause). Thus, interpretation is necessary, and this has been hinted at within the exegesis.

Al-Nakha'i, al-Hasan, 'Isa, and Khalid read it as uqti-tat (أُقْتِتَتْ) with a hamza and a light qaf. Abu al-Ashhab, 'Amr ibn 'Ubayd, Abu 'Amr, and also 'Isa read it as wuqti-tat (وُقْتِتَتْ) with a waw, following the root, because the hamza is a substitution for a waw that carries an original damma (vowel point), which is a consistent rule as explained in its proper place. 'Isa said that wuqti-tat is a dialect of the lower tribes of Mudar. 'Abdullah ibn al-Hasan and Abu Ja'far read it as wuqitat (وُقِتَتْ) with one waw and a light qaf. Al-Hasan also read it as wuqqitat (وُقِّتَتْ) with two waws on the scale of fuw'ilat. In all of the aforementioned cases, idha (when) is conditional.