Thoughts on Traditional Tatar Calendars and Dating
The Islamic calendar is a lunar calendar whose time reckoning is tied to the Moon phases. Each month lasts for a full lunation, which is the time span from one New Moon to the next, and the Moon cycle encompasses all its phases. The timing of the months in the Islamic calendar is based on astronomical observation. A new month can only begin after a Waxing Crescent Moon is observed shortly after sunset. The Waxing Crescent Moon is the phase that starts right after a New Moon. As a result, a month is defined as the average duration of a revolution of the Moon around the Earth (29.53 days). By convention, months of thirty days and twenty-nine days succeed each other (So says the Tabular System). This requires adding one full day every three years to the annual calendar, i.e., a leap year.
Unlike other calendar systems that use leap days or leap months to synchronize the calendar with the solar year, the Islamic calendar is completely detached from astronomical seasons marked by the equinoxes and solstices. An Islamic year consistently falls about eleven days short of the solar year, thus counting 354 or 355 days and not 365. For that reason, the Islamic calendar cannot be used for agriculture or other activities traditionally linked to the seasons.
The Islamic calendar traditionally has twelve months with twenty-nine or thirty days. If the Crescent Moon is visible shortly after sunset on the evening of day twenty-nine, the following day is the first day of the new month. If no sighting is made, a thirtieth day is added to the current month, which is then followed by the first day of the subsequent month.
The origin of the Islamic calendar is dated 1 Muharram 1 AH and was set to the first new moon after the day the Prophet Muhammad moved from Mecca to Medina (the Hijra). The Julian equivalent is 19 April 622 CE. The calendar did not begin, however, until seventeen years after the Hijra, when Caliph Umr’ entertained proposals to establish a new calendar era. His decision was to begin that era with the year during which Muslims established a new community (umma) in Medina. Once this was decided, the order of the months was debated, with various suggestions rejected for equally various reasons until the idea of continuing the already established order was agreed upon. Hence, Muharram became the month beginning the Islamic year.
To make Islamic time reckoning more predictable and universal, Muslim scholars developed the Tabular Islamic calendar in the eighth century CE. This system uses arithmetical rules to determine the length of each month and inserts leap days on a regular basis. Each year has twelve months. However, their length is predetermined: months with uneven numbers have thirty days, while months with even numbers have twenty-nine days.
There exist different approximate formulas for conversion between the Gregorian and Islamic calendars. Two are as follows:
(1) Islamic to Gregorian: AH = 1.030684 x (CE - 621.5643); Gregorian to Islamic: CE = 0.970229 x AH + 621.5643
(2) Islamic to Gregorian: AH = (CE - 622) x 33 + 32; Gregorian to Islamic: AH + 622 - (AH + 332)
The problem with these is that they do not produce strict correspondence between the years of the two era captured by CE and AH. A better solution is provided by calculators, such as the one found online at https://www.linktoislam.net/islamic-calendar/hijri-date-converter/.
NB. Because the Julian and Gregorian calendars are themselves based upon different calculations as to the actual time it takes the Earth to circle once around the sun, their dating is not in sync one to the other. Thus, when the Gregorian calendar was introduced on 4 October 1582, the day following was reckoned as ten days later than the Julian, or 15 October. Over time, additional days had to be added to distinguish between the two.
"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which calendar reference system is used for the date given. Doing so has been a common practice for those studying Russian History prior to 1 January 1918, when the Gregorian calendar replaced the Julian that had been in use for centuries. The day following was dated 14 January.