When speaking of the day of judgment, Jesus clearly gave evidence of a limitation on his knowledge when he said, "but of that day and hour knoweth no man, no, not the angels which are in Heaven, neither the son, but the Father." Mark 13:32 and Matt 24:36. But God knows all. His knowledge is without any limitations. That Jesus, of his own admission, did not know when the day of judgment would be, is clear proof that Jesus is not all-knowing, and that Jesus is therefore not God.
The only thing clear about this particular rejection is that those who wrote the argument completely ignored the fact that Jesus not only was God, but they have conveniently failed to emphasize his humanity when Jesus made his comment. For if the Muslim would have focused on his humanity in these two particular references, like they usually do when demeaning Jesus' person, then he would not have jumped to the hasty conclusion that he did.
It was not uncommon for Jesus to suppress his deity to allow his human nature to manifest itself to the degree of deficiency we see in the verses cited. Indeed, Jesus did not know the day, just like no man would know. Yet, just because Jesus did not know the day of judgment, as a man, in no way necessitated that he was not God. For early on in Matthew's Gospel we see that Jesus had another name which the Muslim conveniently overlooks, and that is Immanuel. And Immanuel means "God with us" (1:23). And as far as knowledge is concerned, we read in John's Gospel that not only did Jesus know all men (2:24), but that he "knew what was in man" as well (2:25). That's not bad for someone who allegedly was not God.