Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The Muslim calendar is lunar. The beginnings and ends of the lunar calendar are determined by the various positions of the moon.
To elaborate more on the questions you raised, we would like to cite this fatwa published on www.islamweb.net,
First of all, the start of the lunar month is not calculated based on the Gregorian calendar as you suggested, but on the sighting of the new moon (crescent). Allah Almighty says, “They ask you, (O Muhammad), about the new moons. Say, “They are measurements of time for the people and for Hajj.“ (Al-Baqarah 2:189)
The Prophet (peace and blessings be upon him) said, “The year is twelve months, four of which are sacred. Three of them are in succession – Thul-Qi`dah, Thul-Hijjah and Al-Muharram, and (the fourth is) Rajab of (the tribe of) Mudar, which comes between Jumada and Sha`ban.” (Al-Bukhari)
The Prophet (peace and blessings be upon him) also said about the month of Ramadan, “Observe fasting with its sighting and break it with its sighting.” (Al-Bukhari and Muslim)
This adequately underlines the method of determining the beginning and end of a lunar month.
As for the unification of the Islamic Hijri calendar all over the world, nothing in the Qur’an or the Sunnah explicitly states that the beginnings and ends of the lunar months should be unified all over the world.
Rather, Muslims should determine the beginning and end of the lunar months by the sighting of the moon.
Whenever Muslims see the moon of Ramadan in any part of the world, they must observe fasting, and those who do not see it should not observe fasting until it is seen.
It should be also known that the differences in the sighting of the moon have been common and acceptable throughout the history of Islam and that the matter is open for ijtihad (personal reasoning by competent scholars).
The differing on this matter is tolerable within the Muslim Ummah and each has evidence to support his view.
The fact that the beginnings and ends of the lunar months depend on the sighting of the moon does not mean that the sun has nothing to do with calculating time. Rather, both the sun and the moon are both considered in calculating time. Allah Almighty says, “(He is) the cleaver of daybreak and has made the night for rest and the sun and moon for calculation. That is the determination of the Exalted in Might, the Knowing.” (Al-An`am 6:96)
Commenting on this verse, Imam Al-Baghawi (may Allah have mercy on him) said, “It means that the positioning of the sun and the moon is considered in calculating time. If it were not for the day and night and the sun and moon, we would not be able to calculate time.”
Allah also says, “It is He who made the sun a shining light and the moon a derived light and determined for it phases – that you may know the number of years and account (of time).” (Yunus 10:50)
At-Tahir ibn`Ashur said, “The verse refers to calculating the beginning and end of months based on the positioning of the moon, because the pronoun in the verse refers to the moon. The seasons, on the other hand, are determined by the positioning of the sun.”
Without doubt, determining the time of Prayers depends on the positioning of the sun and not the moon. Zhuhr Prayer is due when the sun is at its zenith in the middle of the sky, `Asr is when the shadow of everything is equal in length to the object itself, Maghrib is at sunset, and so on.
However, the beginning and end of the lunar months are determined by the sighting of the crescent and the number of days and the duration and length of days and nights depend on the sunrise and sunset.
As for the question about whether the Qur’an refers to the solar (Gregorian) calendar, some scholars of Qur’an exegesis (Tafseer) have stated that the reference to the three hundred and nine years mentioned in the verse (18:25) actually means three hundred plus nine years in lunar years, which is three hundred years in solar years.
Allah Almighty says, “And they remained in their cave for three hundred years and exceeded by nine.” (Al-Kahf 18:25) Ibn Kathir commented on it saying, “The length of time was three hundred plus nine years in lunar years, which is three hundred years in solar years.”
Almighty Allah knows best.
Excerpted with slight editorial modification from, www.islamweb.net