The Gregorian date (Anno Domini/Common Era) is the present Christian/World solar calendar based on birthdate of Christ.
The current Persian/Iranian calendar is also a solar calendar. Originally the Jalali calendar (Anno Jalali) began on 15 March 1079,
instituted by the Seljuk Sultan Jalal al-Din Malik Shah I. This was replaced in 1925 by the Shamsi Hijrah calendar
(S.H.) which takes its origin from the year, 622 CE, in which Mohammad left Makkah for Madinah, the ‘Hijrah’.
The law enacting it was passed under the early Pahlavi dynasty.
The first day of the year is set as the first day of spring in “the true solar year” as it has been in the
Jalali calendar. Each day begins at midnight and each week begins on Saturday and ends on Friday. The Spring equinox
occurs at the time when the sun is exactly overhead at the equator but this can be before or after midday at the Iranian
Standard Time longitude (52o 30' E). If before on that day, new year’s day is that day; if after, new
year’s day is the following day. So years may vary in length by a day more or less in the twelfth month. The first
six months of the year have 31 days each, the next five, 30 days each and the last month 29 or 30 days.
Ahmad Birashk’s proposal for a calculated system of leap years
There are no leap years rules as in the Gregorian Calendar but in 1993 Ahmad Birashk proposed a complex
mathematical formula with a pattern repeating every 2820 years. His average year length works out to
365.24219852. The actual solar tropical year is 365.24219878 days so his calndar is accurate to 1 day
in 3.8 million years! The authorities never took this up and still rely on astronomical observations to
determine the moment of the astronomical vernal equinox.
This date conversion calculation is relevant to when the Gregorian calendar was adopted on Friday, 15 October 1582 and
the modern Iranian calendar adopted on Tuesday 31st March 1925 CE.