Sexagenary cycle is a cycle of sixty terms used for recording days or years, which was used to record years from around the middle of the 3rd century B.C. It is important to understand ancient Chinese chronology.
A Chinese era name (年号/年號) is the title based on the numbering years in an emperor's reign. It originated as a motto or slogan chosen by an emperor, and some emperors have several era names. The numbering of the year increases on the first day of the Chinese calendar each year. For example, 138 BC was the third year of Jianyuan (建元), since 140 BC was the first year, thus 138 BC was also as 建元三年.