In its note on Mark 13:32, The NKJV Study Bible says:
The NKJV Study Bible - Mark 13:32
13:32 As one who was fully God and at the same time fully man, Jesus possessed all the attributes of deity, including omnipotence and omniscience. He knew what was in people's hearts (2:8) and He could still the waves (4:39). When Jesus became a man, however, He voluntarily placed certain knowledge in the hands of the Father (Phil. 2:5-8). Of course today, glorified in heaven, Jesus now knows the day and hour of His return.
The above note is confusing, because it begins with saying that "Jesus possessed all the attributes of deity, including omnipotence and omniscience". It then cites Jesus knowing what was in people's hearts (Mark 2:8), an example which, although it would be consistent with Jesus knowing all things, falls well short of establishing anything like omniscience.
However, it then proceeds to say that "He voluntarily placed certain knowledge in the hands of the Father", and seems to imply that knowing the day and the hour was part of this knowledge, finishing with, "today, glorified in heaven, Jesus now knows the day and hour of His return".
This is simply a contradiction. If Jesus was "fully God", then as the first part of the note says, He was omniscient. But someone who is omniscient cannot "(place) certain knowledge in the hands of the Father" to the point where they say they do not "know" it. As soon as knowledge is placed outside of their immediate access, they have ceased to be omniscient. So, the NKJV Study Bible is seemingly affirming that Jesus is omniscient, but then acknowledging that He did not know certain information. This appears to be a confused expression of a modern Kenosis view of the Incarnation, which acknowledges that Jesus actually lacked divine attributes such that when He said He did not know something, for instance, He actually did not have access to that information.
However, these Kenosis theories are novel in the realm of Trinitarian theology. Such a view would have been rejected by the classical commentators which are responsible for the venerable Trinitarian tradition which the NKJV Study Bible is attempting place itself within. This is because they recognized that if Jesus is the Being known as God, then He knows with certainty all that God knows, because He possesses that mind. If their minds diverge, then they are different Beings. And, God cannot cease to be omniscient. If God could give up attributes, and then incarnate, what has incarnated is no longer God. Consider the implications of this - if God could give up omniscience, omnipotence, omnipresence, and so on, to "incarnate" into another form, then what has incarnated is definitely not "fully God", otherwise, the term "God" has lost its meaning, and has become arbitrary. It no longer describes the maximally great First Cause, it describes some derivation of that entity, stripped of its essential attributes.
Therefore, if God the Son gave certain knowledge to the Father when He incarnated such that He lost it, then the Son changed in His essential nature, and ceased to be God. Also, in this case, there would be a single Being, God, with two Persons who have omniscience, and one who lacks it. Yet, all would be called one "God", and the same Being. This is not a coherent position to hold.
If, however, the Son did retain omniscience, then Jesus could not say that He did not know the day and the hour of His return, because in His divine nature, He certainly did know it. In this case, He would have had to say something like, "My human mind does not know the day and the hour, but the mind of the Trinity does". But, He does not say this. Instead, He names a Person - the Father - and says that "my Father only" (Matthew 24:36) knows the information, and without qualification, says that He does not.
In every case, the explanations of this passage which attempt to affirm that Jesus is God run into problems. This passage, taken straightforwardly, is a clear confession of the Lord which distinguishes Him from God, and neither He, nor the Gospel authors, expend any effort in qualifying what He says in any way.