Dual hour: shí

Each shí (時; 时) was 1⁄12 of the time between one midnight and the next,^[2] making it roughly double the modern hour. These dual hours are named after the earthly branches in order, with midnight in the first shí. This first shí traditionally occurred from 23:00 to 01:00 on the 24-hour clock, but was changed during the Song dynasty so that it fell from 00:00 to 02:00, with midnight at the beginning.^[2]

Starting from the end of the Tang dynasty into the Song dynasty, each shí was divided in half, with the first half called the initial hour (初) and the second called the central hour (正).^[2] The change of the midnight hour in the Song dynasty could thus be stated as going from the central hour of the first shí (子正) to the initial hour of the first shí (子初).

One-hundredth of a day: kè

Days were also divided into smaller units, called kè (刻). One kè was usually defined as 1⁄100 of a day until 1628, though there were short periods before then where days had 96, 108 or 120 kè.^[2] kè literally means "mark" or "engraving", referring to the marks placed on sundials^[4] or water clocks^[5] to help keep time.

Using the definition of kè as 1⁄100 of a day, each kè is equal to 0.24 hours, 14.4 minutes, or 14 minutes 24 seconds. Every shí contains 81⁄3 kè, with 7 or 8 full kè and partial beginning or ending kè. These fractional kè are multiples of 1⁄6 kè, or 2 minutes 24 seconds.^{[lower-alpha 1]} The 7 or 8 full kè within each shí were referred to as "major kè" (大刻). Each 1⁄6 of a kè was called a "minor kè" (小刻).^[6]

Describing the time during daylight

Both shí and kè were used to describe the time, through one of two ways:

1. Eight kè mode. Before the Tang dynasty, the shí were noted first, then each of the major kè were counted up to 8.^[6]
   1. As an example, counting by major kè from the first shí to the second: zǐ (子), zǐ yī kè (子一刻), zǐ èr kè (子二刻), zǐ sān kè (子三刻), zǐ sì kè (子四刻), zǐ wǔ kè (子五刻), zǐ liù kè (子六刻), zǐ qī kè (子七刻), zǐ bā kè (子八刻), chǒu (丑).
   2. The time xū yī kè (戌一刻) would be read as "1 kè after xū shí", making the time 20:09:36.
2. Four kè mode. After the Tang dynasty's division of the shí, it was still noted first, but with an added description of which half of the shí the kè was taking place in. Since this narrowed the range of the possible major kè down to four, it was only necessary to specify the major kè between one and four.^[6]
   1. This changes the first example above to: zǐ initial (子初), zǐ initial 1 kè (子初一刻), zǐ initial 2 kè (子初二刻), zǐ initial 3 kè (子初三刻), zǐ initial 4 kè (子初四刻), zǐ central^{[lower-alpha 2]} (子正), zǐ central 1 kè (子正一刻), zǐ central 2 kè (子正二刻), zǐ central 3 kè (子正三刻), zǐ central 4 kè (子正四刻), chǒu initial (丑初).
   2. The time sì central 3 kè (巳正三刻) would be read as "the third kè in the second half of sì", corresponding to the time 11:31:12.

Smaller time units

One-tenth of a day: gēng

Gēng (更) is a time signal given by drum or gong. The drum was sounded by the drum tower in city centers, and by night watchman hitting a gong in other areas. The character for gēng 更, literally meaning "rotation" or "watch", comes from the rotation of watchmen sounding these signals.

The first gēng theoretically comes at sundown, but was standardized to fall at yǒu shí central 1 kè, or 19:12. The time between each gēng is 1⁄10 of a day, making a gēng 2.4 hours—or 2 hours 24 minutes—long.

The 5 gēngs in the night are numbered from one to five: yì gēng (一更) (alternately chū gēng (初更) for "initial watch"); èr gēng (二更); sān gēng (三更); sì gēng (四更); and wǔ gēng (五更). The 5 gēngs in daytime are named after times of day listed in the Book of Sui, which describes the legendary Yellow Emperor dividing the day and night into ten equal parts. They are morning (朝); midmorning, (禺); noon, (中); afternoon (晡); and evening (夕).^[9]

As a 10-part system, the gēng are strongly associated with the 10 celestial stems, especially since the stems are used to count off the gēng during the night in Chinese literature.^[9]

One-sixtieth of a day: Diǎn

Diǎn (点; 點), or point, marked when the bell time signal was rung. The time signal was released by the drum tower or local temples.

Each diǎn or point is 1⁄60 of a day, making them 0.4 hours, or 24 minutes, long. Every sixth diǎn falls on the gēng, with the rest evenly dividing every gēng into 6 equal parts.

Describing the time during the night

Gēng and diǎn were used together to precisely describe the time at night.

Counting from the first gēng to the next would look like this: yìgēng (一更), yìgēng 1 diǎn (一更一点; 一更一點), yìgēng 2 diǎn (一更二点; 一更二點), yìgēng 3 diǎn (一更三点; 一更三點), yìgēng 4 diǎn (一更四点; 一更四點), yìgēng 5 diǎn (一更五点; 一更五點), èrgēng (二更).

Given the time sāngēng 2 diǎn (三更二点; 三更二點), you would read it as "two diǎn after sāngēng", and find the time to be 00:48.^{[lower-alpha 4]}

The night length is inconsistent during a year. The nineteenth volume of the Book of Sui says that at the winter solstice, a day was measured to be 60% night, and at the summer solstice, only 40% night.^[10] The official start of night thus had a variation from 0 to 1 gēng.

This variation was handled in different ways. From the start of the Western Han dynasty in 206 BC until 102 AD, yìgēng was moved back one kè every 9th day from the winter solstice to the summer solstice, and moved forward one kè every 9th day from summer solstice to the winter solstice.^[10] The Xia Calendar (夏历; 夏曆), introduced in 102 AD, added or subtracted a kè to the start of night whenever the sun moved 2.5° north or south from its previous position.^[10]