This converter does not work for dates before 1912 AD.
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The calendar system currently in use by the United States and much of the world is the Gregorian calendar. This calendar, based upon the length of the Earth's revolution around the Sun (and hence called a solar calendar) was instituted in October 15, 1582 AD by Pope Gregory XIII as a reform to the previously used Julian calendar. Under the Gregorian calendar, a solar year is divided up into 12 months of 30 or 31 days (with February having 28 or 29 depending on if the year is a leap year). This gives a year of 365 or 366 days. However, the true period of the Earth's revolution around the sun (measured from vernal equinox to vernal equinox) is 365 days, 5 hours, 48 minutes and 46 seconds.
Over time this discrepancy would cause the official calendar to be out of sync with various celestial events such as the equinoxes and solstices. To rectify this, a series of leap years were added into the calendar. On a leap year an extra day is added to the end of February, February 29th. According to the Gregorian calendar, leap years occur in every year divisible by 4, execept those that are divisible by 100 but not 400. So 1900 was not a leap year, but the year 1996 was a leap year and 2000 will be. These leap years keep the calendar in sync with the solar year to an accuracy of about 1 day in 2500 years.
This system can be extrapolated backwards to produce dates previous to 1582. In that case, 1 BC is a leap year, as is 5 BC and so forth. My calendar converter can do this extrapolation back to 4713 BC, which is the start of the Julian period, explained below.
| Month | Days | Month | Days |
| 1. January | 31 | 7. July | 31 |
| 2. February | 28 or 29 | 8. August | 31 |
| 3. March | 31 | 9. September | 30 |
| 4. April | 30 | 10. October | 31 |
| 5. May | 31 | 11. November | 30 |
| 6. June | 30 | 12. December | 31 |
Before the Gregorian calendar, the Julian calendar had been in use since it was instituted by Julius Ceasar around 45 BC. The principal difference between the Julian and Gregorian calendars is the frequency of leap years. Under the Julian system, every fourth year is a leap year. This causes an error of about 1 day every 128 years. When the Gregorian calendar was begun in 1582, this error had accumulated to 10 days. Hence, the last day of the Julian calendar, October 4, 1582 AD, was followed the next day by the first day of the Gregorian calendar, October 15, to rectify this error. Of course, it is possible to extrapolate the Julian calendar forward and the Gregorian calendar backward, to find a correspondence for days on either calendar, as the above converter does.
The Julian period is sequence of days starting at day 0 on January 1, 4713 BC of the Julian calendar. Each day after this can be associated with a unique Julian day. This system is useful in astronomy and provides a useful starting point for conversion amongst different calendar systems. The Julian day for December 31, 1996 is 2450449. Be careful not to confuse the Julian period with the Julian calendar.
The Chinese agricultural calendar is also partially solar though because 7 times in a 19 year cycle, an extra leap month (runyue) is be added to the year to bring it back into line with the longer solar year.
To explain the basis for determining when leap months are added, one must first understand the Chinese system of solar terms. 24 dates, made up of 12 principal terms and 12 sectional terms, divide the solar year into 24 periods that are based on the earth's position around the sun. These include the equinoxes and the solstices. According to the Chinese calendar, the winter solstice must occur in month 11 of the year. A lunar month in which a principal term does not occur becomes a leap (or intercalary) month and is assigned the number of the month that preceded it but is designated as a leap. If this happens to occur twice in one year, only the first month in which it occurs in a leap month. The Chinese new year itself starts on the second new moon after the winter solstice.
Chinese years, months, and days are also assigned a name based upon the Chinese system of the heavenly stems and earthly branches. In this cyclical system, each year, month, and day is associated with one of the 10 heavenly stems and 12 earthly branches. Each successive time period will have a new stem and branch, until going through the stems 6 times and the branches 5 times, to give 60 unique combinations. In the case of years and dates, this gives a continuous cycle for thousands of years. This is similar for months, but in the case of a leap month, it is assigned its previous month's branch/stem combination with the leap designation added. This is why the combination is so easily calculated for years and days, but requires tables or complicated astronomical calculations to find months.