Old Calendar (旧暦)
The old calendar refers to a calendar previously used when a new one is introduced. The calendar after calendrical reform is called "a new calendar."
The old calendar in many countries refers to the one used before the Gregorian calendar which is now currently used.
East Asia
In many countries in East Asia the Chinese calendar or its derivatives were used before the Gregorian reform. These calendars are classified as lunisolar. The old calendar is therefore called "a lunisolar calendar," which is however incorrect. If that is the case, the new calendar is called "a solar calendar."
Calendars were reformed many times over before they were changed to the Gregorian calendar. Such changes were minor in comparison with the Gregorian reform. Thus, when detailed calendar rules do not matter, the former calendar rules are often summarized as the old calendar.
The old calendar in each country is essentially the same although the standard time is different. That is why "saku" (the first day of a month) or "sekki" (24 divisions of the solar year) moves because of time differences, resulting in a day's or month's gap. In 2007, for example, the saku fell in Japan and China a day later on February 18 before dawn (Japan local time) but it fell in Vietnam on February 17. For this reason the Chinese New Year's Day (January 1 in the old calendar) fell on February 18 in both Japan and China whereas it fell on February 17 in Vietnam. This type of shift happens once in several years although it does not cause great confusion, except in January where the Chinese new year may shift, as it is resolved on the first day of the next month (in the old calendar). A standard time used on the old calendar may differ from the country's standard time.
Japan
Japan's old calendar is Tenpo-reki (calendar). As mentioned later, however, the old calendar as currently used is slightly different from the Tenpo calendar prior to the calendar reform.
The Tenpo calendar was used until December 2, the 5th year of Meiji (in the old calendar) (December 31, 1872). The following December 3 (in the old calendar) was changed to January 1, the 6th year of Meiji (1873) as the Gregorian (solar) calendar was introduced. This calendar reform was announced on December 9, 1872 and was implemented in the month that followed. It was implemented with such a short notice because the Meiji government wanted to avoid paying its officials monthly wages, whose system was introduced after the Meiji restoration, 13 times a year (as the coming year in the old calendar had leap June). The old calendar, commonly known as the lunar or old calendar, is still used in fortune-telling and traditional events.
For details of confusion caused by the reform, see the section on 'Gregorian Calendar: the Implementation of the Gregorian Calendar in Japan'.
The old calendar was calculated based on the apparent solar time in Kyoto until the Edo period. The time and dates of its 24 divisions and first days of months required for its calculation were developed according to empirically known constants and cycles. This is why calculation was made every year to determine the addition of a leap month or long and short months (a long month had 30 days and a short month 29 days) which annually shifted.
The time zone used in the modern old calendar is Japan local time (UTC plus 9) and is almost equal to the mean solar time at latitude 135 east. Kyoto, on the other hand, lies at 135.46 degrees east in latitude, which corresponds to UTC plus 903. The equation of time further causes a maximum time difference of plus or minus 15 minutes. The position of planets is calculated using the equations based on astrodynamics (position astronomy). Thus, the modern old calendar may cause some dates earlier or later by a day than, or fall in different months from those in the Tenpo calendar during the Edo period.
China, Korea and so on.
The Qing dynasty (the present People's Republic of China, Taiwan, and Mongolia) as well as the Joseon dynasty (the present Republic of Korea and Democratic People's Republic of Korea) used the Shixian calendar established by the Qing dynasty in 1644. In the Korean peninsula the Gregorian reform was implemented in 1896 and in China it was introduced in 1912 when the Republic of China was established.
The old calendars in these regions are totally the same. They are calculated using China local time (UTC plus 8) regardless of local standard times, and this applies to Korea whose standard time is UTC plus 9 as well as Chinese societies all over the world.
In China the old calendar is often called "the Chinese calendar" and in this case the new calendar is called "official calendar." Other appellations of the "Chinese calendar" are "the old calendar," "the lunar calendar," or "the old six calendars."
In Korea the old calendar is often called "the lunar calendar" otherwise "the old calendar.")
It is called "the Chinese calendar" in English.
In these countries, unlike in Japan, the traditional festivals and annual events are usually celebrated according to the old calendars. The Chinese New Year in particular is celebrated much more than the Gregorian counterpart. It is called "Spring Festival" in China, "Seollal" in Korea. It is a national holiday in China, Taiwan and Korea as well as Singapore, Malaysia and Indonesia that have a significant number of people of Chinese origins. In China the Spring Festival, the Duanwu Festival, and the Moon Festival are national holidays based on the Chinese calendar. In Korea the Mid-Autumn Festival (August 15 in the old calendar) and Buddha's Birthday (April 8 in the old calendar) are celebrated.
In both China and Korea the birthday is expressed in the old calendar. However, some people use the new calendar in specifying their birthdays, so that it is sometimes unclear which calendar is used when we are given only the date of someone's birthday.
In Mongol the old calendar originally meant the Shixian calendar which, however, is seldom used at the present. Widely used by people and for religious rituals before and after the Gregorian reform, the Mongolian traditional calendar which is a modified version of the Kalacakra calendar that originated in India is recognized as similar in function to the old calendars in other countries. The Kalacakra calendar was never used as official calendar though it was occasionally presented as the old calendar.
Vietnam
The old calendar in Vietnam is almost similar to the Chinese calendar. Some dates, however, may differ as it uses Vietnam local time (UTC plus 7). The way the old calendar is regarded in Vietnam is quite similar to China and other Asian countries. The Vietnamese New Year (Tet) is a big holiday. The old calendar in Vietnam is referred to as the Vietnamese calendar internationally.
Western Countries
In the West the old calendar (old style) refers to the Julian calendar before the Gregorian reform. This also applies to some countries in the new continent where the Julian calendar has never been used. The Gregorian reform was implemented individually in each country between 1582 (in France, Italy, Portugal and Spain) and 1924 (in Greece).
In English "Old Style" or "O.S." next to a date indicates that the date is based on the Julian calendar.
In Germany a different territorium reformed its old calendar at a different time, so that both old and new calendar dates were written side by side or the name of a city was added to a date, and such customs remained for some time.
In Rome the Roman calendar which was the lunar (solar) calendar was used before the Julian calendar.