Vietnamese era names were titles adopted in historical Vietnam for the purpose of year identification and numbering.
Era names originated in 140 BCE in China, during the reign of the Emperor Wu of Han. Since the middle of the 6th century CE, independent Vietnamese dynasties started to proclaim their own era names.
Chinese era names, also known as reign mottos, were titles used by various Chinese dynasties and regimes in Imperial China for the purpose of year identification and numbering. The first monarch to adopt era names was the Emperor Wu of Han in 140 BCE, and this system remained the official method of year identification and numbering until the establishment of the Republic of China in 1912 CE, when the era name system was superseded by the Republic of China calendar. Other polities in the Sinosphere—Korea, Vietnam and Japan—also adopted the concept of era name as a result of Chinese politico-cultural influence.
The Japanese era name (Japanese: 元号, Hepburn: gengō; "era name") or nengō (年号, year name), is the first of the two elements that identify years in the Japanese era calendar scheme. The second element is a number which indicates the year number within the era (with the first year being "gan (元)", meaning "origin, basis"), followed by the literal "nen (年)" meaning "year".
Era names originated in 140 BCE in Imperial China, during the reign of the Emperor Wu of Han. As elsewhere in the Sinosphere, the use of era names was originally derived from Chinese imperial practice, although the Japanese system is independent of the Chinese, Korean, and Vietnamese era name systems. Unlike its other Sinosphere counterparts, Japanese era names are still in official use. Government offices usually require era names and years for official papers.