The International Civil Calendar is an adaptation of the Indian National Civil Calendar for international use. Primarily, the names of the months are changed to more generic names for more universal use. Like the Indian National Civil Calendar, the International Civil Calendar assigns the longer months to those seasons of the tropical year that are currently longer. Those months of the tropical year when the Sun is usually North of the celestial equator are the Boreal months, while those months when the sun is generally south of the celestial equator are the Austral months:
|Generic name||(Abbr.)||Alternate name||Indian name||Length (days)||Usual Gregorian start date|
|Boreal1||(B1)||Ekaboreal||Chaitra||30 (31)||March 22|
To keep the month lengths aligned as closely as possible to the slowly changing lengths of the seasons, the month lengths should be shifted one whole month every 1700-1800 years. The current lengths should have begun use in 1100 AD/CE, and should be shifted by a month in 2800 AD/CE.
Currently, the Indian National Civil Calendar uses the same leap day rules as the Gregorian Calendar, which keeps them both aligned. However, a 33-year leap day rule would track the year to year equinoxes more accurately. Such a leap day rule could be truncated every 400 years to keep the calendar aligned with the Gregorian Calendar, such that both would have the same average lengths and number of days in 400 years.