In Juuni Kokuki, Ono sensei writes the pregnancy duration is ten months. This depends on not only lunar months but also counting from 1 instead of 0. In Juuni Kokuki world, they often use East Asian age reckoning system about characters’ age. So, I think they use old Japanese counting system in other ways as well.
When we read 十月十日 as Jūgatsu Tōka, which means October 10. But, Totsuki Tōka (十月十日) means ten months and ten days, and also means the normal pregnancy duration. Sometimes, today’s Japanese people say it is longer than the true pregnancy duration. But I don’t think so.
I will write my opinion about this, though a lot of texts about it already exist on the Internet. The opinion is not academic but experiential. Shall we start?
First, you think of yourself as a Japanese woman who lived in more than 150 years ago.
She doesn’t know her ovulation days and doesn’t have thermometers nor pregnancy self test kits. She uses a traditional Japanese lunar-solar calendar, so in her mind a month basically has 29 or 30 days alternately.
If she is in such circumstances, how does she know her pregnancy by herself? Probably, by her morning sickness or by missing her period. When she has got her pregnancy, she would recall her last period. If she is a healthy woman, the last period should be within a few months when she knew her pregnancy. And then, she counts months of her pregnancy. At this time, she must think the first day of her last period as the first day of the pregnancy. And she thinks the month is the first month of the pregnancy.
According to counting system from 1 instead of 0, Totsuki Tōka (十月十日) is approximately 280 days. For example, imagine the first day of her last period is October 13. October is the first month by counting from 1 instead of 0. Then the tenth month is July and July 23 is her expected delivery date. As pointed out above, in the old Japanese calendar, a month basically has 29 or 30 days alternately.
Now, we try to calculate the number of days from October 13 to July 23.
It is 29 days x 4 + 30 days x 5 + 10 days = 276 days or 29 days x 5 + 30 days x 4 + 10 days = 275 days.
This is very close to 280 days.
As you know, the pregnancy duration is vary considerably from person to person. So, Totsuki Tōka (十月十日) was a very practical and useful way in the past.