How to ID a film's Japanese release date from its poster?

[Miles Wood]

I have many, many Japanese posters and nearly everyone has a number by
which it's possible to date the release (or re-release) of a particular
film...simply take the first two numbers and add 25. At least that was
the case upto 1989, I believe, when everything changed, and now it seems
to me much more random. Here's a few examples:
GODZILLA VS MOTHRA bears the no. 92503 obviously indicating a 1992
release. Simple enough.
SPIRITED AWAY bears the number 20312 (do the last two numbers 12 added
to 89 = 2001?)
SCARLET DIVA is 13356 (do the first two numbers added to 89 = 2002?)
Finally Nic Ray's THEY LIVE BY NIGHT is 37581 - does this indicate it
was a 1981 release?

Is there any method to this madness?

Thanks!

Miles