I used the table with all the funny numbers in KM for the funny numbers. Just like Arabic numbers, Japanese/Chinese numbers are easy to remember with a bit of practice. The date conversion method is in the introduction of KM, but it works just like any ole regnal year anywhere in Asia. Since the dates of the eras are in the header of the Japan chapter, that's a short cut. The eras are specified in the header of the Japan chapter in KM. The country name is in Wikipedia, in the Japan lemma. I am now waiting with trepidation for Martin's comment, because he's a pro when it comes to Japanese. He doesn't call himself translateltd for nothing.
Peter