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)