In the past week all my calendars have been showing 23 heisei and I cannot get rid of it.

Don't know if you still have the problem,

Reason: Manufacturing settings

Calendar type

"Japanese" selected

but the way to solve the problem is

Tap in settings

Tap General

down to International tap

Calendar Tap

Select Gregorian

Cheers,😉

I Have this problem on my calendar my notes and a few other places. The name or the word Heisei appears in the description or title.