It was a bittersweet ending for sure, but at least the author wrapped up the loose plot threads pretty decently (considering it was a time travel story and all).
(While I doubt something official would come out to confirm/deny it (so basically is relegated to headcanon), but based on the jump back to 10 years in the future for Hanada, theoretically they could get back together since it seemed he still remembered her to some degree deja-vu like. It's not that huge of age gap.)
Also, just realized the other work the author did (which I also really liked), I knew something seemed familiar about the art style and narrative style.