I'm sure he's using the "day with the Lord is a like a 1000 years and 1000 years like a day" verse. But even if that is the case, fixing the day of the flood is practically impossible. Most biblical scholars think the flood happened about 4000 years ago, not 7000 years.
I thought the 7000 years went like this. 2000 years from Adam to the Flood. 2000 years form the Flood to Jesus. 2000 years from Jesus till now. That makes for 6000 years. The next 1000 will the the 1000 year reign of Christ, which would correspond to the 7th day of the week, the day of rest. But before you start the 1000 year reign, you have to get through the 7 year Great Tribulation.
The population of Mankind will be greatly reduced after the 7 year tribulation. But there will another 1000 years (Rev 21-22), after which, man will again turn from God, and there will be a final war and a final judgement. Then the universe melts.
We should have at least 1007 years left before the universe melts and a new heavens and a new earth is created. But the Great Tribulation could start almost any time.
Does his calculations account for leap years?
:)
The problem in fixing the exact date of the flood, is in the Genealogies after the flood. I believe the time from Adam to King Solomon is straightforward. But setting the time for King Solomon is difficult. But I believe it's banded by a few hundred years. Certainly not the 2500 years required to put the flood 7000 years ago.