(6:96) It is He who cleaveth the daybreak (from the dark). He made the night for rest and tranquillity and the SUN AND THE MOON FOR THE RECKONING (OF TIME).
(10.5) It is He who made the Sun the source of light and the Moon a reflected light and MEASURED OUT THE STAGES FOR HER THAT YOU MAY KNOW THE NUMBER OF YEARS AND THE COUNT (OF TIME). This is nothing but a part of His constructive design.
(55:5) The Sun and the Moon follow courses (EXACTLY) COMPUTED.
(9:36) Lo! The number of months with Allah is TWELVE months by Allah’s ordinance in the day that He created the Heavens and the Earth.
(2:189) They ask thee concerning the New Moons. Say: they are but signs to mark FIXED PERIODS OF TIME in the affairs of men, and for Pilgrimage.
Thus, the Qur’an advocates the use of both the Sun and the Moon to reckon time. The question that may be raised is how to integrate both on the same calendar. If we look at some of the published Islamic calendars then it is obvious that this has already taken place. The basic Islamic calendar that is produced nowadays is a solar calendar with the dates of the start of Lunar months and important dates included. Unfortunately, no unified procedure is adopted to print the Lunar months and the two Eids. In all cases regarding the start of Ramadan and the Eids, there are qualifying small prints at the bottom, which state: “Subject to the visibility of the Moon”, and this causes great dissension among the Muslims.
The uncertainty caused by adhering to the visibility of the Moon causes enormous difficulties for people in various parts of the world and different walks of life. For example, in the U.K in the year 1992, Ramadan started on three different dates resulting in 31 days fasting in some cases! Likewise Eid was held on three different dates. Additionally, the policy of not calculating the date in advance sometimes prevents people in some walks of life from taking a day off for Eid celebrations even though they may be entitled to do so.