Responses
Omg are you kiyo irl, because you're very curious and doing a research over a manga haha jk. I love your analysis though
Your comment has resurrected my soul thank you for doing all that research and then sharing it with us!
From the previous comments, and a lil google search, there was the Great Kanto earthquake on September 1st, 1923 (Tokyo-Yokohama Earthquake) and caused an estimate of 140,000 deaths.
Tokyo and Yokohama are very close to each other on the map. So yeah, Kiyo and Ozaki were probably affected. However, since Kitsune can only die from certain things*, Ozaki most likely survived the earthquake.
The Japanese school year ends in March. Since Kiyo started school in 1920, he would graduate in 1923 March and start looking for a job in Tokyo to stay with Ozaki.
September 1st, 1923 in the Japanese Calendar is a Saturday (I think it's the same calendar globally but my dumbass searched Japanese calendar just in case). The earthquake started 11:58 pm.
Since Kiyo graduated and probably found a job/is looking for a job, he should be working normal work hours and presumably, at that time, is at home with Ozaki (in bed cuddling/doing the you know -u-). So even if the earthquake affected their area, Ozaki sensed the earthquake (Remember, he's a Kitsune and has fox senses, so he probably could hear the earthquake even in his sleep) and protected Kiyo.
One of the comments mentioned that Ozaki looked sad in the extra for Chapter 5, but that was in 1922, before the earthquake. Again, I don't know whether or not the Author deliberately set this manga right before the earthquake, but in my mind, Kiyo did not die and continued to live on with Ozaki (preferably forever but the author just HAD to bring up the fact that Kiyo will die in 30 - 40 years I'm not crying you are).
*I tried searching up how Kitsune can die. There were a variety of answer, from cutting off all its tails, starvation, taking away its hoshi no tama, etc. I didn't know which one was right but I just assume Ozaki was pretty much invincible hehe.