In the second year of the Yuanhe era under Emperor Zhang of Han (85 CE), the court determined the old calendar had too many errors and decreed the adoption of the new Sifen Calendar.
This reform officially established the sixty-year Ganzhi cycle for counting years, termed "One Cycle of the Azure Dragon." Prior to this, Ganzhi was primarily used for counting days (since oracle bone times) and months.
This was a watershed moment: it established the official status of Ganzhi year-counting, which has continued unbroken to this day. This marked the complete construction of the Year-Month-Day Ganzhi calendar system, providing a solid foundation for ancient destiny analysis centered on birth year.