How is Matthew 24:36 explained by trinitarians?

No. It is not contrary. This going to be a little pedantic, but I'm going to try to get you to look at the more pertinent issue: the person of Christ.

The two things you asked about don't directly have much to do with each other. The comparison is incongruent. What I suppose you are asking is that because Christ did not know something that God the Father knows, then is God divided?

To obtain proper perspective on this, you have to make some decisions in your line of reasoning. Your decisions influence your answer. My primary assumption is that the scriptures agree perfectly with each other, even in cases when we cannot immediately understand the harmony. In this regard, we must keep in mind the opening chapter of John.

In John, we read that in the Beginning was the Word. The Word with God and the Word was God. We also read that the Word came in flesh. That's the entire point of Christ: God, the one who made everything, came in flesh. Being God, He has all qualities of God, including omniscience. Yet, there is something He admits not knowing. So the proper question is not whether or not this verse is contrary to trinitarianism but rather "is it contrary to the claims of Christ being God, or is there disagreement between Matthew and John?"

Why ask it this way? Because there's nothing in our language and understanding of the trinity that directly says that in order for a person to be part of the trinity, he must be all-knowing. Instead, what we have is transitive understanding of omniscience being required in order to be in the trinity: you aren't in the trinity if you aren't God, and God is omniscient.

And so the answer is: either Christ was all-knowing, or else he was not. If not, then he was not God... except that John says he was. So if he did not know the day or hour, then he was not God. If he did know the day or hour, then it seems that Matthew's quote was wrong.

And here is the fundamental point: The scriptures claim both; Christ is God and Christ didn't know something. The problem seems to be that there is something incongruent about the person of Christ.

In history, the church has opted not to call this incongruence a contradiction but rather a mystery, one that is abstracted by the hypostatic union. We can only conjecture how Christ did not know something yet was still able to be called God. It's not even that there aren't good theories about how this is possible, it's just that it is indeed conjecture because the scriptures don't speak to it.

This must be understood of Christ as the son of man, and not as the Son of God.

As Son of God he knew all of God’s purposes and designs; for these were purposed in him. He knew from the beginning who would betray him, and who would believe in him; Otherwise how Jesus, who knew so correctly all the other particulars like not one stone should be left on another, should be ignorant of this day and hour? How he in whom the fullness of the Godhead dwelt bodily, and all the treasures of wisdom and knowledge, should not know this small matter. How could the Deity which dwelt in the man Christ Jesus might, at one time, communicate less of the knowledge of futurity to him than at another? So he must know also the day of the last judgment, since it is appointed by God, and he is ordained to execute it: but the sense is, that as he, as man and mediator, came not to destroy, but to save; so it was not any part of his work, as such, to know, nor had he it in commission to make known the time: but his Father only; to the exclusion of all creatures, angels and men; but not to the exclusion of Christ as God, who, as such, is omniscient; nor of the Holy Spirit, who is acquainted with the deep things of God, the secrets of his heart, and this among others.

It has been also said that the verb rendered "knoweth" means sometimes to "make" known or to reveal, and that the passage means, "that day and hour none makes known, neither the angels, nor the Son, but the Father. They suppose the verb οιδεν to have the force of the Hebrew conjugation Hiphel, in which verbs are taken in a causative, declarative, or permissive sense; and that it means here, make known, or promulgate. This intimates that this secret was not to be made known, either by men or angels, no, not even by the Son of man himself; but it should be made known by the Father only, in the execution of the purposes of his justice.

I'm not sure if this is too simplisitic, but I think the answer is that although God the Father, Son and Holy Spirit are all equally God, this does not mean that they are all the same being. We see this when Jesus came to earth, not God the Father; God the Father and Jesus send the Holy Spirit at pentecost - they are not the Holy Spirit.

We see in the Garden that Jesus and God the Father have two different wills:

"Father, if you are willing, take this cup from me; yet not my will, but yours be done." (Luke 22:42 NIV)

If all three are seperate and have individual will, then why do we need to say that they all share the same knowledge?

I think in order to understand things like this it is necessary to come to terms with how Jesus operated in his earthly ministry.

According to Phillipans 2:5 ff:

Christ Jesus, 6 who, though he was in the form of God, did not count equality with God a thing to be grasped, 7 but emptied himself, by taking the form of a servant, being born in the likeness of men. 8 And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross.

Traditional, orthodox Christian teaching is that Jesus, when on earth, was fully God and fully man at the same time. In order to attain our salvation, the Godhead determined that the act of disobedience in the garden of Eden would be countered by an equal act of obedience. To that end, by his own divine fiat Jesus, God the Son, chose to limit himself to an exercise of power equal to that which an obedient human could exercise. That is, he emptied himself of his equality with God and for a time operated out of a limited human power.

All that he knew while in this humbled state he knew because the Father revealed it to him through the Holy Spirit. That is why we can do "these things and greater", because he operated in the same power and revelation that the Holy Spirit grants the church. As he himself said:

So Jesus said to them, “Truly, truly, I say to you, the Son can do nothing of his own accord, but only what he sees the Father doing. For whatever the Father does, that the Son does likewise (John 5:19).

In theory at any time, Jesus could have shrugged off the mantel of humanity and resumed his Godly state; but had he done so our salvation would have been lost, perfect obedience would have been broken and the unity of the Trinity would have been shattered. That last is why I say, in theory, because the area of understanding the dual natures of the Son is a deep mystery that the church can only explore and probe, but never fully understand until we see him in glory in the next life. Then the great mysteries of God will be revealed to us.

So when Jesus said that he, the Son, did not know the day or the hour of his second coming, he meant only that as the Son of Man (i.e. Jesus in his limited state during his earthly ministry) the Father had not revealed to him the time of his return, only that he would return.