June 21, 2012
Kuwada figures out how modern clocks tell time in chapter 30 and chapter 32 of Serpent of Time.
Timekeeping in Japan during the Middle Ages was based on the Chinese "lunisolar" system and the Chinese zodiac. Calendrical years also moved through a cycle of 10 "Heavenly Stems" and 12 "Earthly Branches." This is where the "year of the [zodiac sign]" convention comes from.
As illustrated here, the zodiac was also used in hourly timekeeping.
Intercalary or leap months were necessary to make the solstices line up. When Japan switched the Gregorian calendar early in the Meiji period, a whole month was lopped off to accommodate the change (the date jumped from November to January). The months were simply numbered one through twelve.
Vestiges of this switch are still found in the date of the Obon holiday, Japan's main summer festival. Obon is "officially" celebrated on the 15th of August (according to the old lunar calendar), but in the Tokyo region it is often celebrated on the 15th of July (the adjusted solar date).