That has bothered me as well (setting aside omniscience....i.e., forgetting what would have already been known).What does it mean to be "omniscient?" To know all things, past, present and future? So when scripture says Jesus knows all things (John 21:17) that must mean Jesus is "omniscient." So when Jesus says He does not know the time of His return, that means He is not omniscient. Sounds like we are being double minded.
I see another ploy being hoisted, that God the Father is omniscient, knowing everything imaginable, but Jesus put some of His omniscience in a lock box while incarnate. But God said "now I know" when testing Abraham which means God, no matter the Person, is all powerful enough to put omniscience in a lock box. (Genesis 22:12)
I think sometimes we have to take Scripture as it comes without demanding it fit into any paticular model of human understanding. Jesus is God. Jesus did not know the hour.