Japan came into contact with the Chinese calendar very early, but started calculating it and using fully about the beginning of the 7th century.  This calendar, or a modification of it, was used by Japan until 1873, when it adopted the Gregorian (modern Western) calendar as the official calendar.  However, as almost all source materials for events in Japan before 1873 use the Japanese calendar, as do Japanese historians, this Wiki uses the Japanese calendar in principle. (See SamuraiWiki:About dates.)
Dates in Japanese are given as, for example, 10th month, 18th day (十月十八日). Though months do have names, in fact several names (though they are hardly ever used now), numbers are the principle month designations.
The Japanese calendar was "behind" the Western calendar by three to seven and a half weeks, depending on the year, so if one just wants to get a feel for the season based on the Japanese date, it is probably adequate to think of a date one month later. For example, an event of 7/15 happened on Aug. 15, give or take a few weeks (i.e. from the beginning of August to early September). However, to construct a chronology, more must be known.
Characteristics of the Chinese calendar
The Chinese calendar, at least by the time it was adopted in Japan, was not an observational one ("There, we can see the new moon, so the month has started."), but a predictive one, based on calculations, as was necessary for a widely-spread out, developed bureaucracy. The calculation methods were revised at various times, both in China and Japan, so not everything below applies to all periods, especially the way of determining intercalary months. Further, as a result of Chinese and Japanese authorities performing their calendrical calculations separately, the Chinese and Japanese calendars often differed historically by a day or several.
The Chinese calendar is a lunar, actually a luni-solar, calendar. The start of the month is determined by the new moon, but the position of the month in the year is determined by the solar year, that is, course of the seasons. One should note that although in the the West seasons are considered to begin on the equinoxes and solstices, in the Chinese calendar, the start of spring (立春) is exactly half-way between the winter solstice and the spring equinox, and similarly for other seasons.
Among the characteristics of the Chinese-type calender are the following:
- Months start on the day of the astronomical new moon, that is when the sun and moon are aligned and solar eclipses can occur.  (In contrast, the Ancient Near Eastern calendars started the month the evening after the crescent moon was seen, several days after the astronomical new moon.)
- Months are either 29 (Shô no tsuki 小の月) or 30 days (Dai no tsuki 大の月) long. This means that the date 2/30 is possible, whereas 3/31 is not. Unlike some other calendars, it is not the case that a particular month is normally a particular length. There is no way of knowing whether a month is 29 or 30 days long except by looking at the calendar for a particular year. Normally, but frequently not, 29-day and 30-day months alternate.
- The calendar also takes into account the solar year. The solar year is divided into twelve periods by twelve solar points (中気 chûki) : the winter solstice, the spring equinox, the sumer solstice, the autumn solstice, and two points between each.  The months are numbered according to the solar point that occurs during that month. For example, the month in which the winter solstice occurs is the 11th month, and the one in which the spring equinox occurs is the 2nd month.
- Because the lunar month is less than 1/12 of a solar year, sometimes a month falls entirely between the solar points (中気). In such cases the month is called an intercalary (閏 uruu) month, and is given the same number as the previous month. For more, see below on Intercalary Months.
- The first month (New Year) is the month that has the second solar point after the winter solstice. Thus New Year Day is usually the day of the new moon nearest to the first day of spring, about Feb. 4 (see above on the starts of seasons). In the present Japanese modern lunar calendar, New Years Day is between about Jan. 21 and Feb. 19.  This means dates towards the end of the Japanese year are in the next year of the Western calendar. For instance, the death of the Twenty-Six Martyrs of Japan was 1596/12/19, but it is better known by the Western date of Feb. 5, 1597. The Akô rônin carried out their revenge on Genroku 15 (1702).12.15, which was Jan. 31, 1703 , so one sees both 1702 and 1703 as the date.
- Sometimes the start of the month was moved to achieve a particular purpose such as to avoid four long months in a row.
As the solar year is about 365.25 days but 12 lunar months last only about 354, each year the months start about 11 days earlier with respect to the solar calendar (the seasons) as they did the previous year. In fact, in about two years and eight months, the calendar moves a full month ahead. In order to keep the lunar and solar calendar from getting too far apart, as noted in the characteristics above, when a month finishes too soon, the month is given the same number as the preceding month, but is called an "閏 (uruu)" month. For example, if there are two "third months," the first is called just "third month," but the next is called the "uruu third month". In this Wiki it is referred to as "int. 3", for "intercalary 3rd month". Thus, 1599/3 was followed by 1599/int. 3, which was followed by 1599/4. This type of intercalary month, which can happen any month in the year, is a feature of the Chinese-type luni-solar calendar.
It probably would have been easier to put the intercalary month at the end of the year like western luni-solar calendars as the ancient Babylonian and Jewish calendars did. There is some evidence that the Chinese early used that system too. But apparently, the Chinese preferred the current method because it tied the calendar to the even solar points.
Besides the month-day method of indicating dates, cylic dates, dates using the sexegenary cycle were also used supplementally. They can in some ways be compared to the modern days of the week. The days are given a designation in the cycle, such as mizunoto i, and the cycle repeats without a break for anything--even such as the beginning of months or years. For example, 909/2/27 was a 癸亥 (no. 60 in the cycle) day, 909/2/28 was a 甲子 (no. 1) day , 909/2/29 was a 乙丑 (no. 2) day, and the following day, 909/3/1, was a 丙寅 (no. 3) day. This is like the use of the cycle for year dates, but whereas cyclic year dates were used for dating into the Meiji period, the use of the cycle for days was much more restricted. They are used in the Nihon Shoki and in some other early chronicles, though not for the Kojiki. In the Azuma Kagami, a Kamakura period chronicle, the cyclic day is given in the heading, but not in the text under the heading, as 廿日庚寅...去る十二日... (Twentieth Day, Kanoe-tora [of 1192/7]...On the previous 12th...).
However, I have not seen dates given in cyclic days most other places I have looked--the Heian Period literary works, a few Muromachi Period graves and steles, Azuchi-Momoyama Period letters and Edo period bureaucratic letters and records. Many of them give the cyclic year. But letters read like, "Your letter of the past 21st reached here the 27th," or "I am leaving the coming 24th, so accompany me," with no mention of the cyclic date in the text or in the letter date.
That is not to say that the cyclic days, especially the shi of the days were not important in daily life. For example, the Heian Period had many customs and superstitions related to cyclic days (Morris's The World of the Shining Prince). A tree in a shrine was planted "in the Hour of the Bird, on the Day of the Bird, in the Year of the Bird." Edo period pocket calendars indicated the cyclic designations of some days. One custom that has survived to the present is that a pregnant woman should first put on, or at least purchase, a maternity girdle on a day of the dog. However, few calendars now include the cyclic days.
Unlike conversions between era years and Western years, which is a fairly easy calculation (see Year dates and Japanese Eras), conversion between the Western and Chinese calendar can only be done using a monthly table based on historical documents. One table is Tuchihashi's. However, now there are several websites that provide calculators which look up dates. One is the following: http://web.me.com/meyer.eva/www.yukikurete.de/nengo_calc.htm which will give the Western year for the Japanese date.
Here are several notes on its use:
- "Tsuchihashi" and "Zölner" are the authors of two calendrical tables. For most years they are identical, but in some cases, especially for early years, they are not. To see the problem years use the "database" button and use the link given there.
- To use the calculator you have to know the nengô of the year. An explanation of how to do this is found in Japanese eras and the Year dates pages. Or, even easier, run down the list of nengô in the pull-down menu on the calculator until you find the period your year is in. The year in the nengo system is [Western year] - [Nengo start year] + 1. For example, 1600 is in the Keichô period, which started in 1596. 1600 - 1596 + 1 = 5, so 1600 is Keichô 5. Unfortunately all numbers are in kanji, but you can easily learn the kanji or just count down on the pull-down menu. Note that the intercalary months are preceded by 閏.
- Before entering the month and day, the Western date shown is that of New Year Day of the Japanese year.
- For dates Tenshô 10/9/18 (Oct. 4, 1582) and before, dates are in the Julian calendar, but from the next day, Tenshô 10/9/19 (Oct. 15, 1582) the dates are the Gregorian calendar. This change in the western calendar means you cannot just blindly use the program; you need to know which calendar to use for your western date.
- From 1873 on, the Japanese dates are the Gregorian calendar.
- As this is a European site, the Western dates are given in the order D-M-Y, so 2.6.1465 is June 2, 1465, not February 6.
- There is no calculator on that site for getting from Western to Japanese. So the best way is to pick a Japanese month one month before (like the 9th month for an October date) and then use the "try and then adjust" method several times.
- The traditional date is 604. E. Reishauer, et. al., East Asia: The Great Tradition (Boston: Houghton Mifflin, 1958), p. 476.
- A Japanese lunar calendar is still published and can be found in almanac-calendars and daily newspapers. However, it is ignored by most people. It is similar, but not identical to the modern Chinese lunar calendar.
- The calendar used in Japan in the seventh century used the average length of lunar months, but later ones tried to calculate the exact moment of the new moon.
- The year is also divided into 24 "solar terms" (節気 sekki) such as the "Great Cold." These are the 12 chûki plus the 12 points that are half-way between chûki.
- After 1844 the method of calculating the solar points changed, and there could be two in one month. In Japan in such cases the months with the solstices and equinoxes were given their traditional month numberings, and the numbers of other months were arranged to fit.
- Because both the western calendar and the Japanese calendar have changed in relation to the solar year over time, the possible dates vary with the period. For example, towards the end of the 16th century the new year started between Jan. 24 and Feb. 24.
- The Islamic calendar does not have intercalary months, so any given month changes its season year by year. There is no way of knowing the season of any month except by checking on a chart of that year.
- The Nihon Shoki uses a highly roundabout way of expressing days of the month. Instead of saying, for instance, [670 AD] 夏四月三十日壬申 (summer, the 30th day [mizunoue-saru] of the fourth month) it uses 夏四月癸卯朔壬申 (the 壬申 day in the 4th month, the first day 朔 of which is a 癸卯 day). Since 癸卯 is the 40th day of the cycle and 壬申 is the 9th day of the cycle, one can calculated that the 壬申 day is the 30th day of the month.
- Catholic countries in Europe used the Gregorian immediately; some of Holland used it soon; the English changed in 1752; and Russia in 1918. Because of the distance from Europe, the Jesuits in Japan probably heard about the change in July 1585. To change from Gregorian into Julian, in the 16th and 17th century go back 10 days; in the 18th century, 11 days ["Give us back our eleven days" was the cry when the change was made in England]; in the 19th century, 12 days, and in the 20th and 21st century, 13 days.
- Kôjien Dictionary
- Ôsaki Masatsugu大崎正次,"Nihon Jasokai no Kaireki"「日本耶蘇會の改暦——日本年代學への一機構として」Chiri Rekishi『歴史地理』Vol.70, No.4、1937.