The Archetypes Calendar
This is an accurate lunisolar calendar with mostly 10-day 'tweeks' (ten-day-weeks) which approximates the Chinese Calendar. It was invented by Peter Meyer in March-April 2010 and slightly revised by him in July 2011.
Each year has 12 or 13 months (13-month years are called 'long'), and each month has 29 or 30 days. All odd-numbered months have 30 days and all even-numbered months have 29 days, except that in some years ('leap years') the 10th month has 30 days. A long year may also be a leap year.
ARC years are grouped into consecutive periods of 1,803 years called ARC periods. The ARC years 443 through 2245 constitute an ARC period. The Julian day number of the first day of this ARC period is 897,474 (which establishes the correlation with the Common Era Calendar).
The rules for long and leap years are:
A year with position p (1,2,3,...) in an ARC period is a long year if and only if (664*p + 901) mod 1803 < 664.
A year with position p is a leap year if and only if (350*p + 901) mod 1803 < 350.
A month consists of three consecutive tweeks. The first two always have 10 days, while the third may have 9 or 10 days, for a total of 29 or 30 days in a month. A complete definition of this calendar, along with names of the months and the weeks, and various properties of the calendar, is at The Archetypes Calendar.
This calendar has interesting connections to the Chinese Lunar Calendar. The range of new year's days of both calendars in the Common Era Calendar is the same (namely, January 20 through February 21). It is almost always true that a year in the Archetypes Calendar is a long year if and only if the corresponding Chinese year has 13 months.
The day and month numbers of dates in the two calendars for all 365,618 days over the range 4400-01-01 ARC (= 1702-01-28 CE) through 5400-12-29 ARC (= 2703-02-07 CE) were compared, and it was found that when intercalary months in the Chinese Calendar are ignored 50.78% of the days had the same day and month numbers in both calendars, for example, 79-28-09-24 CHL = 4709-09-24 ARC (= 2011-10-20 CE).
This is interesting because the Chinese Calendar is based on the determination of the exact times of dark moons and solar terms by means of complicated astronomical calculations, whereas the Archetypes Calendar is based on the fairly simple rules given above.
Software for converting between the Archetypes Calendar, the Gregorian Calendar and the Chinese Lunar Calendar is described here and here.
See also Messages to CALNDR-L re the Archetypes Calendar and the Chinese Calendar.