Already during the Ottoman period (1299-1923) various arithmetical schemes were used for computing the religious lunar calendar. A comprehensive and easily accessible history of the development and use of these methods during this period is unfortunately still lacking.
During the last decades of the Ottoman period the lunar calendar was apparently based on a tabular calendar based on a 30-year cycle with 19 short years consisting of 354 days and 11 long years consisting of 355 days. In each year the months were successively 30 and 29 days in length, where the last month (Zilhicce) was 29 days in a short year and 30 days in a long year. In each 30-year cycle, the years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26 and 29 were long while the others were short.
Between 1344 AH (which began on 22 July 1925) and 1399 AH (which ended on 20 November 1979) different rules were adopted, the details of which are as yet unclear.
Between 1400 AH (which began on 21 November 1979) and 1435 AH (which ended on 24 October 2014) the computed lunar calendar was based on the following rule:
Since 1 Muharrem 1436 AH (25 October 2014) the computed lunar calendar appears to be based on the following rule:
Based on these criteria the Turkish Presidency of Religious Affairs (Diyanet İşleri Başkanlığı) provides calendrical tables (currently up to 2022 CE / 1443/44 AH) on which this web site is based.