Does the documentation for when they got married really matter? What if they just say "yeah, sure, we'll file it as yesterday" and then just don't. Will anyone ever notice, or does Japan put your date of marriage on your credit card and everything?
I was thinking the same thing. If you had a marriage ceremony without an official there to document it, or gave birth to a child at home, it’s just hearsay what date it actually happened. They probably also mark the day the paperwork was received, but that’s just for clerical reasons. If you come into a government office and say “I got married yesterday, I want to do the paperwork now”, surely that yesterday date would be there somewhere?