An ad hoc (literally, "to this" in Latin) argument is one created to respond to one particular objection.
"And it was the third hour, and they crucified him."
"And it was the preparation of the passover, and about the sixth hour: and he saith unto the Jews, Behold your King! But they cried out, Away with him, away with him, crucify him. Pilate saith unto them, Shall I crucify your King? The chief priests answered, We have no king but Caesar."
Apologists sometimes respond that there are two different time keeping systems. For an example, see "Was Jesus crucified in the third hour or the sixth hour?" There is no Biblical evidence to justify this claim. This argument is an ad hoc speculation invented for the purpose of refuting this particular argument.