They don’t know.
Archaeologically it has been dated around 7,300 cal. BP during the earliest stage of the Jōmon period,[14] but it had also been uncorrected radiocarbon dated to as recently as 6,500 BP.[15] The current accepted calibrated dating adjustment from multiple other sources is between 7165 and 7303 years before present.[1][2][3]
https://en.wikipedia.org/wiki/Akahoya_eruption
What’s 800 years between friends.
Either way, I think if the Japanese can get over it, we should too.
;)