I'm not sure that the kingdom of God is the Millennium. Christ was seated at the right hand of the Father, but the Millennium hadn't started yet (unless you use the ascension and session as the starting point). Since the Millennium in Rev. 20 is explicitly described as the resurrection of the saints after they'd been killed by the beast for not taking his mark, I don't think this is a very good candidate.
The phraseology of Revelation 20:4-6 has been interpreted variously. "This is the first resurrection" (in my opinion) means, "This is the age of the first resurrection" (i.e., the thousand years corresponds to the time when people experience the first [spiritual] resurrection, which is rebirth in Christ), just as Christ's words, "in the resurrection" (Matt.22:30) refers to the era of physical resurrection. If "the millennium" means "a long period of time during which God's people live in the power of a spiritual resurrection" (i.e., now), then its beginning would be when disciples received the Holy Spirit—which, in the case of the apostles, seems to have been on the Sunday afternoon following Christ's resurrection (John 20:22).
We might not be able to nail down (or at least wouldn't agree on the conditions of) the ending point. But, I find it impossible that the kingdom promised throughout scripture has no starting point described or predicted in it.
Matt and Douglas got around to saying what I intended to say before I did. I too was thinking of David—did his kingdom begin when he was anointed by Samuel (corresponding to the baptism of Jesus)? or when the first 400 men acknowledged him as their commander when he fled from Saul (corresponding to Jesus acquiring disciples)? or when Saul was killed and David proclaimed king over Judah (corresponding, perhaps, to Satan's defeat at Calvary)? or when Ishbosheth was killed and David ruled all of Israel (possibly corresponding to Christ's universal reign over the earth)? Couldn't one use any of these as a beginning point, depending on which phase of David's rule was under consideration?
Likewise, when was America founded? Was it with the first European settlements in North America? or the signing of the Declaration of Independence? or the winning of the Revolutionary War? or the drafting of the Constitution? or the election of Washington as first President?
You see, it is not strange at all to suggest that the kingdom was established in stages, but clearly was already present, in some sense, in the lifetime of Jesus (Matt.12:28; Luke 17:20-21) and in the lifetime of Paul (Col.1:13; Rom.14:17). A further development in the coming of the kingdom in power was the defeat of those who had opposed and crucified Christ, in AD70 (Matt.16:28). One, like myself, who is not full-preterist, could additionally anticipate yet another stage of its coming, when "the kingdoms of this world have become the kingdoms of our Lord and of His Christ" (Rev.11:15).