Leap year This makes it easy to convert dates to or from Gregorian. The Thai solar calendar uses the Buddhist Era (BE), but has been synchronized with the Gregorian since AD 1941. From AD 8 the Julian calendar received an extra day added to February in years that are multiples of 4. The Coptic calendar and Ethiopian calendar also add an extra day to the end of the year once every four years before a Julian 29-day February. This rule gives an average year length of 365.25 days. However, it is 11 minutes longer than a tropical year. This means that the
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 (),