Psalm 90 is key to understanding the principle of "millenial days". This is a psalm written by Moses which propounds this principle of "a thousand years . . . are like a day" (verse 4). The psalm follows the outline of the seven days of Creation and is probably at least one reason why Jewish scholars believed the history of mankind would span 7,000 years. The early church, being largely Jewish, also had this same belief as expressed in The Epistle of Barnabas. The text is from the New International Version of the Bible.

Psalm 90
A prayer of Moses the man of God.
Day One – the beginning, the light of God brought forth
1 Lord, you have been our dwelling place throughout all generations.
2 Before the mountains were born or you brought forth the earth and the world, from everlasting to everlasting you are God.
Day Two – expanse separates that which is above from that which is below
3 You turn men back to dust, saying, "Return to dust, O sons of men."
4 For a thousand years in your sight are like a day that has just gone by, or like a watch in the night.
Day Three – vegetation created
5 You sweep men away in the sleep of death; they are like the new grass of the morning—
6 though in the morning it springs up new, by evening it is dry and withered.
Day Four – the sun and moon created
7 We are consumed by your anger and terrified by your indignation.
8 You have set our iniquities before you, our secret sins in the light of your presence.
Day Five – the birds created
9 All our days pass away under your wrath; we finish our years with a moan.
10 The length of our days is seventy years-- or eighty, if we have the strength; yet their span is but trouble and sorrow, for they quickly pass, and we fly away.
Day Six – mankind created
11 Who knows the power of your anger? For your wrath is as great as the fear that is due you.
12 Teach us to number our days aright, that we may gain a heart of wisdom.
13 Relent, O LORD! How long will it be? Have compassion on your servants.
Day Seven – God rested
14 Satisfy us in the morning with your unfailing love, that we may sing for joy and be glad all our days.
15 Make us glad for as many days as you have afflicted us, for as many years as we have seen trouble.
16 May your deeds be shown to your servants, your splendor to their children.
17 May the favor of the Lord our God rest upon us; establish the work of our hands for us-- yes, establish the work of our hands.

The Epistle of Barnabas expressed the thought of the early church that God would strive with mankind and Satan for six thousand years, and then rest on the seventh thousand. This is also the idea expressed by the millenium as described in The Revelation. God will lock up Satan for a thousand years, and there will be peace. Jesus will reign as King of Kings and Prince of Peace. According to Bible scholars, there has been about six thousand years of human history.

Maranatha!