4 mil secs again... but different date


In the TIME-challenge in chapter 10, I found the date 4 mil secs from today to be sept 16.
In this chapter, I tried the method given in the book dateByAddingTimeInterval with 4 mil secs (instead of 1 mil). But now, it returns the date sept 15.
Did you guys try to fill 4 mil secs in in the message in 12 (page 184 in my ebook, or just above figure 12.2)? And do you get the same date as in chapter 10 challenge, or the same difference I get?
Any ideas why?