People who believe in the "Year-Day Theory" try to make Scriptures say that one day equals one thousand years. They use 2 Peter 3:8 to support their theory which says "one day with the Lord is like a thousand years."
Date setters often use the Year-Day theory to try to predict the day of the Rapture and the Second Coming of Christ by substituting "one thousand years" into passages that use "one day." Some people today are teaching that because the earth was created in 7 days, this will equal 7,000 years of human history. They add the 4000 years in the Old Testament plus 2000 years in the New Testament to correspond to the first 6 days. They say Jesus will return in the year 2000 and set up His Millennial Kingdom for 1,000 years, which will fulfill the 7th day. This is an interesting theory, but is nothing more than speculation. We know that there will be at least 7 years of tribulation on earth, so it appears that the earliest year that the Second Coming could occur would be 2007, which destroys this theory.
Others have tried to plug the Year-Day Theory into Daniel 8:13-14, which says, "How long will the vision about the regular sacrifice apply, while the transgression causes horror, so as to allow both the holy place and the host to be trampled?" He said to me, "For 2,300 evenings and mornings; then the holy place will be properly restored." Some people have substituted years for days in this passage, coming up with 2,300 years, which has also been used to predict the Second Coming.
However Daniel 8:14 says, "2,300 evenings and mornings" which clearly means literal days not years. William Miller plugged the Year-Day theory into this passage to wrongly predict the Second Coming of Christ on October 22, 1844, known as "The Great Disappointment."
If we read the entire verse, 2 Peter 3:8 says, "But do no let this one fact escape your notice, beloved, that with the Lord one day is LIKE a thousand years, and a thousand years LIKE one day." This passage teaches that God exists outside of the realm of time, not that a thousand years are equal to one day. Dont fall into the trap of believing the Year-Day theory.