We get a rough timeline in this chapter on page 11
In the middle panel, the book spine shows 1990–1999
but in the bottom panel, it changes to 1990–2009
This means the part with adult Natsume and Shidou is set 10 years into the future
- Main timeline: 2000
- Future timeline: 2010
By the time Hachigatsu Kokonoka happens, Shidou’s already an adult, so that’s set a few years later