In Chinese history, the dynasty before ROC, such as the Qing, the Ming, the Yuan, and more dynasty, usually used emperor‘s reign title to mark year. For example,
萬曆 15年 (Wanli 15th year, 万历 in simpilified Chinese) was 1587, meaning Emperor Wanli’s 15th on the throne. The marking year way dated from Emperor Wudi in Han Dynasty (122 BC) who initiated the way and whose frist reign title was Yuanshou（元狩）in that year that it was 19th year of his reign. Wudi changed the title after 6 years. In his reign time, he adopted 11 titles.
Before the Ming Dynsty, one emperor ususally adopted one more titles in his reign time, and from the Ming one emperor usually adopted a single title.
In practice and custom, new emperor should use his own title.
The meaning of reign titile bears auspicious.
The marking year way was used by Chinese neighoring countries, such as Japan (now Japan still using this way), Korea and Vietnam.
Though ROC was republic, it inherited the tradition (using the nation's reign title instead of common era). After 1949, People's ROC was founded and started using common era.
If a nation use Chinese emperor's reign title to mark year, it means this nation submits to Chinese emperor and is an affiliate nation . For instance, in the Ming, Korea used Ming's emperor's title and was an affiliate. In Wanli period, Japan invaded Korea and Ming sent to army to help Korea. Ming had the duty of protection.
If a local governor starts using his own reign title, it means he rebels and made his own power. It is felony so that all his family and relatives are about to behead.
