The whole timeline doesn't make sense.
At the end of Naruto The Last at the final scene with Bolt and Himawari you can see Hokage monument and Naruto's face isn't up there and Bolt looks like he's 5 or 6 years old.
In the Gaiden flashback Naruto is already Hokage and it seems like around the time of Sarada and the kids were born.
Here's the timeline as I interpret it
2 years after the war is the last
1 year later naruto marries hinata sasuke marries sakura everyone has *** and gives birth like rabbits
3vyears later himawari is born
3 years later the lasts epilogue happens
Few months later naruto becomes hokage chapter 700 happens and sasuke shows up at the kage meeting we saw in chap 700
6 years later gaiden
Because boruto looks 6 in 700 and everyone graduates the academy at 12 except specail cases however for that to be the case mirai would be nine ( asumas daughter) she looks like she's in her teens so more time than a year may have passed between the last and the children's birth