The answer is found in Old Testament prophecy, confirmed by Jesus, and reconfirmed by the Apostle Paul.
In around 600 BC, Daniel interpreted a dream of Babylonian ruler, Nebuchednezzar.
Daniel said the dream concerned four kingdoms.
The first was the Babylonian Empire, which Daniel said would be followed by a second (the Medo-Persian Empire) and a third kingdom (the Greek Empire).
It was in the days of the fourth kingdom though (the Roman Empire), that Daniel said, “In the days of those kings, the God of heaven will set up a kingdom and that kingdom will never be destroyed.” Daniel 2:44. That means that God established the kingdom of God on earth sometime between 27 B.C. (the beginning of the Roman Empire) and 476 A.D. (its end).
In the days of the Roman Empire, specifically, around 30 A.D., John the Baptist preached “Repent, for the kingdom of heaven is at hand.” Matthew 3:1-2. Jesus followed suit, preaching, “Repent for the kingdom of heaven is at hand.” Matthew 4:17.
Jesus told the Pharisees that the kingdom of God was not coming with signs to be observed, but that the kingdom of God was in their midst. Luke 17:20-22. When Jesus sent out the 70 he told them regardless of whether they were accepted or rejected in any of the cities to which they were going they were to tell the people that the kingdom of God had come near to them. Luke 10:1-12. The kingdom of God could not come near to them unless it went with the 70. Jesus told people that if He was casting out demons by the finger of God the kingdom of God had come upon them. Luke 11:14-20. He did, and it had.
The Apostle Paul, writing to the Colossians, said that Jesus had rescued believers from the domain of darkness and transferred them to the kingdom of God. Colossians 1:13. Paul didn’t say that Jesus would but that He had.
When is the kingdom? The kingdom is now. It has been here for nearly 2,000 years, and it is coming, meaning it’s continuing to manifest until Jesus returns and the Kingdom is fully and finally consummated. GS