Chinese era name
A Chinese era name (simplified Chinese: 年号; traditional Chinese: 年號; pinyin: niánhào) is the regnal year, reign period, or regnal title used when traditionally numbering years in an emperor's reign and naming certain Chinese rulers (see the imperial naming conventions). Some emperors have several era names, one after another, where each beginning of a new era resets the numbering of the year back to year one or yuán (元). The numbering of the year increases on the first day of the Chinese calendar each year. The era name originated as a motto or slogan chosen by an emperor.
How the Era System worked 
Emperor Wu of Han (Han Wudi) was conventionally regarded as the first emperor to declare an era name; however he was only the first to use an era name in every year of his reign. His grandfather and father also employed era names, though not continuously. Han Wudi changed period titles every five years or so, going through a total of eleven reigning slogans during his reign from 140 BC to 87 BC.
Each era name has a literary meaning. For instance, the first era name of Han Wudi was Jianyuan (建元 in pinyin: jiàn yuán), literally meaning "establishing the First". Era names also reflected characteristics of political and other landscapes at the time. Jianzhongjingguo (建中靖國 jiàn zhōng jìng guó), the first era name of Emperor Huizong of Song China, means "establishing middle, peaceful country", reflecting his idealism towards moderating the rivalry among the conservative and progressive parties on political and social reformation. The very first era name of the Qing was significant because it means "[the Manchus possess] the Mandate of Heaven".
The process of era name declaration was referred to in traditional Chinese history texts as jianyuan. Declaring a new era name to replace an old one during an emperor's reign was referred to as gaiyuan (改元 gǎi yuán), literally meaning "change the First".
To name a year using an era name only requires counting years from the first year of the era. For example 138 BC was the third year of Jianyuan (建元), since 140 BC was the first year. When more than one monarch used the same motto, the name of the specific monarch or dynasty has to be mentioned. For instance both Han Wudi and Jin Kangdi picked Jianyuan as their motto. Thus 344 AD was the second year of Jianyuan of the Jin Dynasty (or of Jin Kangdi) whereas 139 BC was the second year of Jianyuan of the Han Dynasty (or of Han Wudi). In traditional literature, one can therefore find references like "the first month of the thirteenth year of Jianyuan" (建元十三年元月).
Almost all era names have exactly two characters. Notable exceptions are from the non-Han Chinese Western Xia Dynasty (1032 - 1227). Of the 33 Western Xia era names, seven have more than three characters. For example,
- Tiancilishengguoqing (天賜禮盛國慶 tiān cì lǐ shèng guó qìng) (1070) "Heaven-given ritualistic richness, nationally celebrated"
- Tianshoulifayanzuo (天授禮法延祚 tiān shòu lǐ fǎ yán zuò) (1038) "Heaven-instructed rituals and laws, perpetually blessed"
Before the Ming dynasty, an emperor often changed his era name as often as he liked. The numbering of the year still increases on the first day of the Chinese calendar each year, regardless of the month in which the era name change took place. For example, Emperor Xuanzong of Tang change his era name, Xiantian (先天, pinyin: xiān tiān) to Kaiyuan (開元, pinyin: kāi yuán) in the twelfth (i.e. last) month of the Chinese calendar. The second year of Kaiyuan (開元二年) began on the first day of the next month (i.e. Chinese New Year's Day); this made the first year of Kaiyuan (開元元年) consist of only the last few days in the twelfth month following the name change.
Ming and Qing emperors generally used only one era name during their reign, and it is customary to refer to Ming and Qing emperors by their era names. When an emperor died, his successor would adopt a new era name but the numbering of the new era name would only begin on the next New Year's Day. For example, when the Kangxi Emperor of Qing acceded the throne in 1661, it was a few days after New Year's Day, so the Shunzhi era continued for the rest of the year, and the first year of Kangxi (康熙元年) would only start on New Year's Day the following year, in 1662. Exceptions to this are the Taichang Emperor of Ming (being dead after reigning for only one month, his successor decreed that the Taichang era name be used for the rest of the year), Zhengtong Emperor of Ming and Hung Taiji of Qing (both used two era names).
An era name was a symbol of imperial power. Declaration of another era name when one was already in use was regarded as a challenge to the current emperor. The existence of more than one era name at a time often reflected political unrest. In addition, using a particular era name was a political act implying recognition of a sovereign's right to rule, and one issue that traditional Chinese historians faced was which set of era names to use when dating a historical event. For example, when the Yongle Emperor of Ming usurped his nephew, the Jianwen Emperor's throne in 1402, he ordered all records of the four-year-reign of Jianwen Emperor to be dated as year 32 through year 35 of the Hongwu Emperor (the emperor preceding Jianwen Emperor), in order to establish himself as the legitimate successor of the Hongwu Emperor.
Era names were also employed (under different naming conventions) in other East Asian countries such as Korea, Japan and Vietnam, mostly because of China's cultural influence. They are still used in Japan. The Republic of China Era, used in China from 1912-1949, and still used in Taiwan, marks years as Minguo (i.e. the Republic), which is usually regarded as an era name. On the mainland, era names were abolished with the adoption of the Common Era at the founding of the People's Republic in 1949.
The Republic of China Era 
An era name could only be declared by the emperor before the Republic of China was established. The Republic of China retains the era system, and uses the name "Republic" (民國) for its official dating. The 1st year of the "Republic Era" was 1912. Therefore, 2011 is "the 100th year of the Republic Era" (民國100年).
Era System versus Western Era System 
While the era system is a more traditional system of dating that preserves Chinese and Japanese culture, it presents a problem for the more globalized Asian society and for everyday life.
For example, even though within the nation people will know what era they are in, it is relatively meaningless for other nations. In addition, while the Government of Japan and the Government of the Republic of China only recognize documents dated in the Era System, their treaties with other countries are in the AD (or CE) system.
In the domestic arena, the Era system presents difficult dilemmas, too. For example, in Japan, it is difficult to keep track of the age of people who were born in the previous era. Also, since the ROC and Japan have adopted the Gregorian calendar, it is difficult to track down the February 29 leap year, more difficult than its internationalized counterparts.
Furthermore, in Japan, in theory it is difficult to mention future dates since it is sometimes difficult to tell whether the current emperor will live long enough for its citizens to use that era name. However in practice documents like drivers licenses and 50-year leases use era dates without regard to this problem.
On the other hand, others suggest that the AD system has too much Christian connotation behind it and it is cultural imperialism that a European system is forced upon other civilizations. However, with globalization, there is a trend with Japan and the ROC being more acceptable to the AD system.
Modern history researchers do not care about era names except for supporting other arguments, such as figuring out the biases and attitudes of a particular historian; however such mottos are useful for dating events that were unique in Chinese history. Most Chinese dictionaries have a comprehensive list of era names, while booklets of more detailed and often searchable lists can be found in libraries.
See also 
- Regnal year
- Emperor of China
- Chinese calendar
- Table of Chinese monarchs
- Japanese Eras
- Korean Eras
- Vietnamese Eras