The calendar was luni-solar. The year consisted of 12 months which each started at sunset when the new moon was first seen. This meant that each month was either 29 or 30 days long but their length would change from one year to the next. The new year started on the first new moon after the vernal equinox. After 19 years the cycles of the moon and the sun re-align and so an intercalary month was added at that time to bring the calendar back in line with the seasons. It would still be out by one day every 342 years (18 cycles) but it is not certain whether this correction was applied.
The Bahai year starts on 21 March and contains 365 or 366 days just as the Gregorian Calendar. Leap years are handled in just the same way. The year consists of 19 months each of 19 days. Month 18 is followed by 4 or 5 intercalendary days which are given to feasting and present giving. The first day of each month is also a feast day. Days are considered to begin at sunset on the previous day.
The names of the months are - Splendor, Glory, Beauty, Grandeur, Light, Mercy, Words, Perfection, Names, Might, Will, Knowledge, Power, Speech, Questions, Honour, Sovereignty, Dominion and Loftiness. The intercalendary days falling between Dominion and Loftiness.
The additional days, five or six depending on whether it was a leap year,
were known as :-
When the calendar was planned it was decided to make the autumn equinox the first day of the year. As the calendar was introduced on 24th November 1793 it was decided that the calendar would retrospectively start on 22 September 1792. This was the equinox and the day the French Republic was founded.
Leap years were intended to be as the Gregorian calendar with the addition of Herschel's rule that years divisible by 4000 should not be leap years. Leap years actually occurred in years 3, 7 and 11. Year 15 would have been a leap year but the calendar ended in year 14.
There was also an attempt to introduce a metric day which had ten hours. Each hour had 100 minutes and each minute 100 seconds. One reason the calendar failed has been mentioned (only one day off in every ten) but the traders did not like it as it made international trading difficult.
Every year which when divided by 900 leaves a remainder of 200 or 600 is a leap year.
This makes 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2800 non-leap years, whereas 2000, 2400, and 2900 are leap years. This will not create a conflict with the rest of the world until the year 2800. This rule gives 218 leap years every 900 years, which gives an average year of 365 218/900 days = 365.24222 days, which is certainly more accurate than the official Gregorian number of 365.2425 days. However, this rule is NOT official in Greece.
Years are counted in the Saka Era which began with the vernal equinox in 79 AD (Gregorian). There are 12 months of 30 or 31 days and 365 or 366 days in the year. Leap years are determined in the same manner as the Gregorian Calendar. The year starts on 22 March (Gregorian) each non-leap year and 21 March in leap years. The names of the months are - Caitra, Vaisakha, Jyaistha, Asadha, Sravana, Bhadra, Asvina, Kartika, Agrahayana, Pausa, Magha and Phalguna.
The Indian Religious Calendar usually has 12 months but may have 13. This is because each month starts with the new moon. Each lunar month is given the name of the solar month in which it begins. When two new moons occur in the same solar month then the two lunar months both have the same name but with adhika placed before the name of the first month. Occasionally a solar month will occur with no new moon. When this happens the name of that solar month will not be used for a lunar month. This is known as a ksaya month. Any year which contains such a month will always also contain a adika month so that the total lunar months will never fall as low as 11. Such years occur between 19 and 141 years apart.