The Persian Calendar consists of a 2820 year cycle. Within the cycle are sub-cycles of either 128 or 132 years. These sub-cycles are themselves divided into a smaller cycles of 29, 33 or 37 years. The fourth year within each of the smaller cycles is a leap year. There are 12 months. The first 6 have 31 days; the subsequent 5 have 30 days. The last month has 29 or 30 days depending if its a leap year or not.

Although the Persian calendar is conceptually, very old, it recently went through a series of reforms to resemble its current form. These reforms occurred in 1925 (Iran) and 1957 (Afghanistan). The current definition of the Persian calendar aligns year 1 to beginning of the Islamic era (year 622 in the Julian calendar). Although the Islamic Calendar begins on the Julian equivalent date of June 17, 622; the Persian calendar retains some of the original calendar philosophy by starting on the spring equinox of 622. Unlike the Islamic calendar (a pure lunar calendar), the Persian calendar is solar.

The Zoroastrian Calendars

Zoroastrianism takes its name from its leader and prophet, Zoroaster of the 6th century BC, who taught in the existence of a monotheistic God. (Judaism is arguably the first religion to teach the existence of a monotheistic God, as Judaism predates Zoroastrianism by several thousand years).

Zoroastrianism came into its height with the Sassanian Dynasty of Persia, whose King Shapur I adopted it as Persia's official religion. (Shapur ruled c. 250 BC). Even though the Sassanian Dynasty was wiped out by the Muslim Arabs with the advance of Islam, the religion remained. As a group, the Zoroastrians have been persecuted and have come to settle in different areas. As such, variations on their calendar have sprung up over time. The net result is that there are 3 Zoroastrians calendars extant today. The general rule of the Zoroastrian calendar is solar. The calendar year is set to the coronation year of the last Zoroastrian King, Yazdegird III of Iran.

Fasli

This calendar has undergone the most recent change. In the year 1906, it was decided to align the calendar with Gregorian. New Year begins on the Gregorian equivalent of March 21. Leap years coincide with the Gregorian leap year rules. Each year consists of 13 months: The first 12 have 30 days each, the last month has 5 or 6 days.

Shenshai

This is the calendar of many Parsee Zoroastrianists. The original design of the Shenshai calendar called for adding an additional month every 120 years. However, it is believed that the additional month was only added once, and only in India. The standard Shenshai year (which in reality is all years), calls for 365 days a year. There are no leap years. Each year consists of 13 months: The first 12 have 30 days each, the last month has 5 days. The Shenshai reject the Fasli notion of a 6 day month (the last month of a Fasli leap year has 6 days), as the number 6 is considered contrary to the teachings of Zoroaster. Yet with a year length of 365 days with no corrections, the calendar over time 'drifts', with the start of the new year no longer falling on the Spring Equinox.

Qadimi

This calendar is very similar to Shenshai. The only difference is based on the starting date of the calendar. The Qadimi calendar is one month earlier than the Shenshai calendar.