"And while staying with them [Jesus] ordered them not to depart from Jerusalem, but to wait for the promise of the Father, which, he said, 'you heard from me; for John baptized with water, but you will be baptized with the Holy Spirit not many days from now.'
"So when they had come together, they asked him, 'Lord, will you at this time restore the kingdom to Israel?' He said to them, 'It is not for you to know times or seasons that the Father has fixed by his own authority. But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth." _ Acts 1:4-8, ESV
We have absolutely no clue. Now there are some who have tried to figured it out, even predicted it's arrival, but no one really knows.
When would Israel be restored? Jesus' response: "It's really none of your business."
When will Jesus return? I believe that Jesus' response is the same: "It's really none of your business."
Of course, I do not believe that he is saying this in an angry way, but rather in a very loving and gracious way. The fact that we do not know when Jesus is returning, I believe, is one of God's greatest gifts to all of mankind. Think about it. If Jesus told us the exact day that he was returning, then hope would be horribly affected by those who would have to face suffering during the time when Jesus was not returning. But because of the fact that it was left open, with maybe some clues to his return, hope is still present as we look forward to Jesus arrival.
He also protects us from laziness. As human beings we have a tendency toward sin, not toward righteousness. The only reason that I am able to live a righteous life (which I struggle with every day), and to be seen by God as righteous is because of Jesus' sacrifice on the cross and the indwelling of the Holy Spirit of God in me. That's it. If Jesus had told us the exact day that he was returning (or take any topic of God's timing and apply it here), we would be tempted to just sit back and wait until that day. And we all know how well we do with waiting.
Jesus kept us in the dark about when, but he definitely did not keep us in the dark about what. What are we supposed to do until his return? What are we so supposed to do while we wait for God to do what he is going to do in this world and in our lives? First, live in the power of the Holy Spirit. Second, be his witnesses all over the world. This is what Jesus made very clear.
So I'm guessing the time that is spent by us trying to figure out God's perfect timing of future events is a waste of time, since those things have not been told us. They really are none of our business. But the mission to share Jesus with a broken world still has as much urgency in it as when Jesus gave it to his first apostles. We answer that call and let God "figure out" when everything is going to happen. We remain faithful to what he wants us to do. We wait for when God is going to get it done.
When? It's none of our business.
What? Let's get to work.