Al-Ahqaf: (34) "And the Day those who disbelieved are exposed..."
{Is this not the truth?}
This is a quotation following an implied verb, and this implied verb is what governs the adverbial of time (the "Day").
{This}
This is a reference to the punishment, as evidenced by His saying, Exalted is He: {So taste the punishment}.
The meaning:
It is a mockery of them and a rebuke for their derision regarding the promise and threat of God, and for their saying: {And we are not to be punished} (Ash-Shu'ara: 138).
(35) "So be patient, [O Muhammad], as were those of determination among the messengers and do not be impatient for them. It will be—on the Day they see that which they are promised—as though they had not remained [in the world] except an hour of a day. [This is] notification. And will [any] be destroyed except the defiantly disobedient people?"