February 29 takes approximately 6 hours longer than 365 whole days. A leap day compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur later than intended in the calendar year. The Julian calendar used in Christendom until the 16th century added a leap day every four years; but this rule adds too many days (roughly 3 every 400 years), making the equinoxes and solstices shift gradually to earlier dates. By the 16th century the vernal equinox had drifted to March 11, and the Gregorian calendar was introduced both to shift it back
Chinese calendar year had 12 months, with a ten-day week intercalated in summer as needed to keep up with the tropical year. The 10 Heavenly Stems and 12 Earthly Branches were used to mark days. A third version is the balanced calendar (). A year was 365.25 days, and a month was 29.5 days. After every 16th month, a half-month was intercalated. According to oracle bone records, the Shang dynasty calendar ( BC) was a balanced calendar with 12 to 14 months in a year; the month after the winter solstice was "Zhēngyuè". The first lunisolar calendar was the Zhou calendar (),