First of all, for those who may have been misled by the title, this is not about romantic dating in Japan but about the systems used for recording the date, specifically the years. Sorry for any confusion.
In 20th century pre-war Japan there were three dating systems that were used: the Western system (Gregorian calendar), the Imperial system, and the era name. The first needs no introduction.
The Imperial system is called Shinmu Tennou Sokui Kigen (神武天皇即位紀元) in Japanese. It is often abbreviated as Kouki (皇紀) or Kigen (紀元) in order to save space since the long version is unwieldy. The full version can be translated as Emperor Jimmu Enthronement Year. This dating system became popular in 1872 when the National Foundation Day was first instituted as a national holiday. The purpose was easy to see. The government wished to bolster the legitimacy of the Imperial line (remember this was only 4 years after taking power from the bakufu) by commemorating the legendary first emperor (Jimmu) and therefore celebrating the current emperor.
This dating system begins with the ascension of Emperor Jimmu to the throne in 660 BCE. An easy way to count is to understand that the year 2600 is 1940 in the Gregorian calendar.
So in addition to the annual celebrations, many commemorative items throughout the year were inscribed with the date using this notation. Here is a belt buckle labeled ‘Imperial Year 2600 [1940].’
Oftentimes the number stands alone with kanji preceeding it. Here is a badge like that:
The number at the bottom, 2592, is the date. It corresponds to 1932. Sometimes the number is written in kanji with no ‘Imperial Year’ prefixed. Here is an example:
At the bottom are four number kanji, read from the right: 2589 (二五八九), which is 1929.
More common is the era name designated by the reigning Emperor. These are fairly well known, but just in case, here are the four since the Meiji Restoration: Meiji 明治 1868-1912, Taisho 大正 1912-1926, Showa 昭和 1926-1989, Heisei 平成 1989-present.
