Calendars are generally of three kinds: lunar, solar, and lunisolar. A lunar calendar follows the Moon's phases, a solar calendar follows the Earth's orbit around the Sun, and a lunisolar calendar combines the two. The Chinese calendar is lunisolar: the new moon falls on the first day and the full moon around the fifteenth, while the 24 solar terms follow the Sun's apparent yearly motion.

A Gregorian year is the time the Earth takes to orbit the Sun, 365 days, 5 hours, 48 minutes and 46 seconds (365.24219 days). A common year is counted as 365 days, and a leap day is added to February every four years. A lunar month is one cycle of the Moon, about 29 days, 12 hours, 44 minutes and 4 seconds (29.5306 days), so a long month has 30 days and a short month 29. A purely lunar year has 354 days, 11 fewer than a solar year. To stay aligned with the seasons, an extra month, the leap month, is inserted every two or three years. Ancient China used the rule of 7 leap months in 19 years. As early as the Zhou dynasty, astronomers divided the Sun's path along the ecliptic into 24 equal parts. Those parts are the 24 solar terms.