The bible answers itself. When there is a question in one area, the answer is in another area, so we don't have to guess what was meant by that, the bible tells us what was meant by in the days of Noah. Luke 17:27 is an example of that, just also read the verse before it. What was meant is that it came on a normal day, when people were doing normal things. Noah warned them for a hundred years, then suddenly the door of the ark was closed and the rain fell. It was too late for them. In the same way, you have to obey God now, cause once Christ returns it will be too late.