In 1661, Yasui Santetsu is a member of the Yasui House of the Four Houses of Go. However, he's beginning to doubt that Go is his true calling. Outside of the Go house, he goes by the name Shibukawa Harumi, and he's fascinated by mathematical and astronomical problems. Specifically, Harumi is pouring all of his energy into creating a new Japanese calender.
The story is based on the actual astronomer and calender-reformer Shibukawa Harumi (1639-1715). In Harumi's time, the Japanese used the Chinese lunar-solar Xuan-ming calendar, which had not been updated in nearly 800 years. At that point, there was already a two-day delay in the winter solstice. He reformed the calendar based on his careful astronomical observations.