In 1873, the Gregorian calendar was introduced to Japan. While the Christian way of numbering years is commonly used in Japan today, a parallel numbering system for years based on the reigns of emperors is also frequently applied (see year converter above).
The year 2000, for example, which happened to be the 12th year of the reign of the emperor, whose posthumous name is Heisei, is called "Heisei 12".
Before 1873, lunar calendars, which were originally imported from China, were used in Japan for many centuries. The lunar calendars were based on the cycle of the moon, resulting in years of twelve months of 29 or 30 days (the moon takes about 29 1/2 days to circle the earth), and an occasional 13th month to even out the discrepancy to the solar cycle of 365 1/4 days, i.e. the discrepancy to the seasons.
Various features of the lunar calendar remain intact in today's Japan. For example, years are still commonly associated with the twelve animals: mouse, cow, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog and pig.
Another aspect of the lunar calendar, that survives into modern Japan, is the subdivision of the calendar into six days (rokuyo), similar to the subdivision of the modern calendar into seven weekdays. The six days are called taian, butsumetsu, senpu, tomobiki, shakko and sensho, and they are associated with good and bad fortune.
Taian, for example, is considered the most auspicious of the six days and ideal for holding business or personal events such as wedding ceremonies, while butsumetsu is considered the least auspicious day, and holding funerals is avoided on tomobiki.