Probably 4 or 5 years but that is confusing given himawari age cause how old does she compared to Bolt?
If this takes place before the movie and bolt has this kitten why didn't he use it earlier cause that would have helped naruto alot.
The thing is if you look at them at best I would say The Hokage one shot happen no more then two years ago at best to when we see boruto again (so when boruto is ten or so and the daughter looks 6~ish). Since none of them look any younger then that when compared to how they look in the movie/manga or the gaiden (the gaiden seems to happen right before they take the graduation exam and the movie at best a year afterwards). The thing is SP is SOOOOOOOOOOOOO lazy they'd just copied the designs for the kids and made them eight years old. So now I don't know. As far as I can tell Bolt and the daughter might as well popped out of hinata's womb look exactly as they do right now.