Deciphering the Japanese Calendar

Throughout the research process, one of the most persistent annoyances was the lack of a concrete way to check dates given according to the Japanese calendar with their corresponding Gregorian dates.  A few prominent historical events such as the Battle of Toba-Fushimi and the Battle of Ueno are usually listed with both dates, since Western observers were on hand to take note of them, but contemporary writers are otherwise terrible when it comes to specifying even which calendar they are using.  During the twenty years that the Japanese were interacting with Westerners on the basis of their old calendar, most people did not seem to find understanding it a matter of grave importance.

When I first started casually looking into the matter, I assumed that some kind history geek would already have built a nice website devoted to the subject, with easy reference calendars for examination and cross-reference.


No matter where I looked or what I read, no one had what I was really starting to need:  a side-by-side comparison of the Gregorian and Japanese calendars.  Finally one day when I was bored and frustrated I decided to take what I had been able to find out about the Japanese calendar and see if I could at least get the months and dates to work out from 1868 until the end of 1872 – when Japan adopted the Gregorian calendar. Four years with several concrete, cross-calendar-referenced dates seemed like a safe place to start.

I used an Excel spreadsheet and three websites:  This site gives you mini calendars for each year with the weeks all laid out with their dates, as well as the dates of the moon cycles – first quarter, full moon, last quarter, and new moon.  I started out by using Excel’s autofill feature to create a basic Gregorian calendar.  Then in another column I went through and listed all the moon cycles.  Since I had read that in the Japanese calendar the new moon marked the beginning of the month, and each month had either 29 or 30 days, this gave me some parameters to experiment with.  For a while I was making good progress just backfilling the dates between the ones I had concrete cross-references for.  But I had two recurring problems.  When did the New Years occur?  And was there any rhyme or reason regulating where the leap months fell?  (Since the months were shorter they needed a 13th month occasionally to round out the year).

Wikipedia:  I know everyone says not to trust Wikipedia, but come on.  If the article has sources, and what you’re seeing agrees with everything else out there, why not?  Here I got most of my concrete cross-calendar dates.  I also got some good information on the Japanese seasons and lucky days.  But still no New Year dates.  I was starting to feel that my project was hopeless.  There were too many places where the months on my homemade Japanese calendar weren’t matching up the way they should.  Then, somewhere (and I can’t remember where), I stumbled across a website that casually mentioned that the dates for the Chinese New Year and the Japanese New Year were the same, because they were both based on the lunar cycle.  I promptly started searching for the Chinese calendar equivalent of  This site was it.  All of a sudden, my problems melted away.  First, I verified each year’s Chinese New Year Date.  These agreed with no more than a day’s difference here and there with the moon cycles I had recorded independently.  Then I examined my problem areas.  Every time there was a discrepancy, the Chinese calendar smoothed it out.  My final doubts were laid to rest when between January and July of 1868 I was left with an odd gap between my cross-calendar concrete dates.  The Chinese calendar inserted a leap month in the gap with the correct number of days.

Banzai!  (And no, that’s not a type of miniature tree.)

Now, I realize that this calendar may still be off by a day or two here and there.  Someday some astronomer will probably call me out on it.  But until then, and maybe even then, I’m not too worried.  It is a calendar that has not been used officially in almost 150 years.

I have not had the time or the inclination to work out this side-by-side calendar for the entire 200 years of Tokugawa rule.  There is really no reason anyone needs to.  I may someday complete it as far back as 1853 – the year Perry’s first squadron of Black Ships arrived – but so far I have simply done the years that have a bearing on the main action of this story.  Keep in mind that the Japanese peasants and people in outlying areas continued to use the old lunar calendar for several years after the adoption of the Gregorian calendar, at least for planning their crops, figuring out lucky days for weddings, and so forth.

I am posting my calendar as a resource for anyone who may find it useful.  Use it, refer to it, read it for curiosity’s sake, I don’t really care.  If you are a research fiend like me and are also writing a book about this time period, please list me in your bibliography.  That’s all.