The 4th Shinobi War ended on Naruto's 17th birthday. Kakashi became the 6th Hokage several months later, so not quite Naruto's 18th yet. The Last came two years later (Let's assume its exactly two years), so Naruto would be 19, and almost 20. At this time, it could some how be before October. Then one year later, Boruto was born. Assuming it's exact, Boruto's birthday would have to be some time before October. "Several" means more than 2, so sometime around January-October, one year after The Last.