We have already refuted these false claims several times. Your teachers have taught you wrong. You are not fighting with Posttribbers, you are fighting with Scripture. Once again you do not provide one single quote to support your claim. That is becoming a pattern with you, and one that is typical of engaging with Pretribbers.
Genesis doesn't say he entered the ark 6 or 7 days prior. It shows he received the instruction then. Let us analyze this.
Genesis 7:4-19
1 And the LORD said unto Noah, Come thou and all thy house into the ark; for thee have I seen righteous before me in this generation.
2 Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female.
3 Of fowls also of the air by sevens, the male and the female; to keep seed alive upon the face of all the earth.
4 For yet seven days, and I will cause it to rain upon the earth forty days and forty nights; and every living substance that I have made will I destroy from off the face of the earth.
5 And Noah did according unto all that the LORD commanded him.
6 And Noah was six hundred years old when the flood of waters was upon the earth.
7 And Noah went in, and his sons, and his wife, and his sons' wives with him, into the ark, because of the waters of the flood.
8 Of clean beasts, and of beasts that are not clean, and of fowls, and of every thing that creepeth upon the earth,
9 There went in two and two unto Noah into the ark, the male and the female, as God had commanded Noah.
10 And it came to pass after seven days, that the waters of the flood were upon the earth.
11 In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.
12 And the rain was upon the earth forty days and forty nights.
13 In the selfsame day entered Noah, and Shem, and Ham, and Japheth, the sons of Noah, and Noah's wife, and the three wives of his sons with them, into the ark;
14 They, and every beast after his kind, and all the cattle after their kind, and every creeping thing that creepeth upon the earth after his kind, and every fowl after his kind, every bird of every sort.
15 And they went in unto Noah into the ark, two and two of all flesh, wherein is the breath of life.
16 And they that went in, went in male and female of all flesh, as God had commanded him: and the LORD shut him in.
17 And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift up above the earth.
18 And the waters prevailed, and were increased greatly upon the earth; and the ark went upon the face of the waters.
19 And the waters prevailed exceedingly upon the earth; and all the high hills, that were under the whole heaven, were covered.
This is (not surprisingly) in agreement with the words of Christ. The focus is the day the flood came. That is the saleint day in view here. According to this, "the selfsame day" Noah entered the ark was expressly "after seven days." It was then "that the waters of the flood were upon the earth."
Jesus said in Luke 17:26-30, “as it was in the days of Noe, so shall it be also in the days of the Son of man. They did eat, they drank, they married wives, they were given in marriage, until the day that Noe entered into the ark, and the flood came, and destroyed them all. Likewise also as it was in the days of Lot; they did eat, they drank, they bought, they sold, they planted, they builded; But the same day that Lot went out of Sodom it rained fire and brimstone from heaven, and destroyed them all. Even thus shall it be in the day when the Son of man is revealed.”
Jesus words support my position and damn your doctrine. They show what that "self-same day" was - "the day" of destruction, thus the 7th day. Christ speaks of “the days of Noe” (plural), speaking of the days that preceded the destruction of all the wicked. He then spoke of “the day” (singular), speaking of the actual day when the wicked were wholesale wiped out. This contrast between “the days” and “day” is common in Scripture and is presented in order that we can distinguish between “the lasts days” of time and “the last day” of time, that ushers in the end. The actual day that Noah entered into the ark "the flood came, and took them all away." It was therefore an individual day that God closed the door of the Ark and by doing so damned all those that were left behind. The same will occur when our ark – Christ – arrives at the second coming.