Diagram of the Five Chronos

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Fire
Autumn: 91 days and 25/80ths The value 25/80 reduces to 0.3125. Adding 91 days results in 91.3125 days, exactly one-quarter of a 365.25-day solar year.
From the Beginning of Summer to the Beginning of Autumn term: "Beginning of Summer" (立夏, Lixia) and "Beginning of Autumn" (立秋, Liqiu) are two of the twenty-four solar terms that mark the start of their respective seasons.
Corresponds to Summer

Metal
Winter: 91 days and 25/80ths
From the Beginning of Autumn to the Beginning of Winter
Corresponds to Autumn

Water
Spring: 91 days and 25/80ths
From the Beginning of Winter to the Beginning of Spring
Corresponds to Winter

Wood
Summer: 91 days and 25/80ths
From the Beginning of Spring to the Beginning of Summer
Corresponds to Spring

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Awaiting division among the four seasons: 18 days, 3 quarters?, and 5 parts each.

Earth
For each of the four seasons, the total calculation is 73 days and 5 parts.

In this system, Earth is the central pivot. By taking approximately 18.26 days from each of the four 91.31-day seasons, the year is divided into five equal phases of 73.05 days, one for each of the Five Elements (Wood, Fire, Earth, Metal, Water). This balances the "Five Phases" within the solar year.

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This diagram, the Wu Chen Zhi Tu (Diagram of the Five Chronos), illustrates the early Chinese scientific effort to reconcile the four physical seasons with the five-element cosmological system (Wu Xing). Because a year has four seasons but five elements, the element of Earth was mathematically integrated by "borrowing" time from the end of each season. This allowed scholars to calculate the "Qi" or vital energy of each element as it governed the calendar.