RELIGIOUS BASES FOR CALCULATION
The religious basis for calculation is that all Muslims agree that the most important and the authoritative source and criteria for determining the Islamic dates is to be found in the Qur'an and Sunna, as elaborated and defined in the Shari'ah. No methodology for moon-sighting or decisions about the beginnings of Islamic months can be implemented with out consulting the religious sources.
A number of verses from the Qur'an speak to this issue:
From Surah Baqarah:
"..... They ask thee (O Muhummed) about the crescent moons. Say: They are signs to mark fixed periods of time for mankind and for the pilgrimage....." (2:189)
From Surah Yunus:
"..... It is He who made the sun a radiance and the moon a light and determined their stations that you might know the number of years and reckoning of time ....." (10:5)
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From Surah Ya-Sin:
".... Neither the sun may reach the moon -- nor night will precede the day.... each in special orbit is swimming around according to it's law ...." (36:39)
From Surah Rahman:
".... The sun and moon follow courses exactly computed ..." (55:5)
Furthermore, the following hadith is the basis for determining the beginning of Ramadan:
According to Abu Hurayra, the prophet(pbuh said: "Fast to its sighting (the Hilal) and break your fast to its sighting ..... and if it is obscured to you by clouds, then count the month of Sha'ban to thirty days." (Bukhari and Muslim)
The Qur'anic passages indicate to us, the creations of Allah (swt), that he has established for our benefit a universe full of signs and tools. Among these is the cosmic clock, whose components include the sun and the moon, which we are instructed to use to reckon time and mark the months and seasons. What is also indicated is that the paths of the sun and moon are precisely determined, and follow courses exactly computed, according to the Divine Law. (Imam and Dr. Abdul Jalil Sajid, Justice of Peace and Director of Brighton Islamic Centre).