This focus on calendars is a stroke of genius. Calendars **are** powerful mechanisms of cultural control. Think about how the international standard calendar for business and commerce is the Gregorian calendar, which ties its start date to Christianity. (People do use other calendars in various places and for various purposes, but the Gregorian calendar dominates for international exchange.) Less so now than in the past, but Sunday is designated a no-work day in accordance with that tradition. And think how the rest day figures for other calendars, too—the Jewish calendar or the Islamic calendar. If you don’t know the proper rest day, you can be in trouble—and this is even if you’re an outsider: things stop. And if you don’t stop—depending on the degree of observance—you might be punished. And if the community gradually moves away from this, it can be perceived by the more-faithful as cultural weakening. Calendrical rot is threatening!
The traditional Chinese calendar is a lunisolar calendar that has complicated, intersecting base 10 and base 12 recurring features and indicates certain days as auspicious or inauspicious for various activities. When you combine it with geomantic principles (powers or traits related to compass directions—feng shui), which happens naturally, as feng shui is tied to the solstices and equinoxes, which are calendrical as well as astronomical occurrences, boom, that’s a whole lot of Chinese folk culture you’ve got—and, like the Chinese writing system, it spread to Korea, Vietnam, and Japan.
In Japan (and probably in other East Asian countries, but Japan’s the one I know about), magical powers were attributed to people who could advise on and manipulate the calendar—something that required some good math skills, what with those mixed number bases and various repeating units. If you’ve ever seen the film Onmyōji, you’ve seen the story of one famous example of such a person, Abe no Seimei. In Ninefox Gambit, this magic translates to the “exotic effects” that can be generated in war, relying on the calendar. These same effects don’t work if the calendar is subverted—beware calendrical rot!
There’s one notable instance in Ninefox Gambit in which the protagonist manipulates the heretics’ calendar to gain a tactical advantage—Buuuuuut I can’t spoil it.
This isn’t a review of the book—I have one of those at Goodreads, covering some of the same territory, but in less detail—it’s more of an appreciation of this one aspect of the book. It’s me saying “I SEE WHAT YOU DID HERE, YOON HA LEE! VERY CLEVER!”