Time frame clues for chapter 0: The 5-yen coin with a hole started being minted in 1948. The glasses look early 1960's to me, but the tarot revival started late 1960 to 1970s (depending on the place). The fountain pen is just 20th century. That heavy barrel pen went in and out of style over the decades. Rotary desk phone (chap. 1) also puts it before the 1980s.
Sorry if this has been addressed later in the comments.
Chapter 19 cemented it in the '70s for me where they meet up in daylight for the row boat date. Wallah on page 2 is wearing a cravat and has long sideburns, the woman next to him is wearing a dress of that era and Yukino is wearing flared pants and platform shoes. I hope to never see Narumis' glasses frames in real life - hideous!
So, the tech says pre 1980s, the fashions pinpoint the '70s.
Makes me realise just how much our day-to-day tech has changed in 50 years. I don't care about clothing fashions recycling, but what tech would you REALLY want to roll back to?