Qur'an says moon is created for Calendar
1. Sun and moon follow Hisab:
"The sun and the moon follow courses exactly computed." (55:05)
2. Phases of the moon are the standard to be used for reckoning of time:
"They ask you about the waxing and waning phases of the crescent moons, say they are to mark fixed times for mankind and Hajj." (2:189)
3. Sun and Moon to be used for reckoning of time:
"It is He who made sun a lamp, and the moon a light and fixed the phases, so that you can count years and Hisab." (10:5)
4. Sun and Moon to be used for reckoning of time:
"And He who made the night for rest and sun and moon for reckoning of time. This is the decree of the Exalted, the All-knowing." (6:96)
5. Moon is the measure of month and year:
"The number of months in the sight of Allah is twelve (in a year) so ordained by Him the day He created the heavens and the earth; of them four are sacred; that is the straight usage.......Verily the transposing (of a prohibited month) is an addition to unbelief: the unbelievers are led to wrong thereby: for they make it lawful one year and forbidden another year in order to adjust the number of months forbidden by Allah and make such forbidden ones lawful. The evil of their course seems pleasing to them. But Allah guideth not those who reject faith." (9:36-37)
6. Ordained orbits of sun and moon and measured phases of moon:
"And the moon, we have measured for her mansions (to traverse) till she returns like the old, (withered, and curved-up like a sickle) date-palm. It is not permitted for sun to catch-up the moon, nor can the night outstrip the day; each swims along in (its own) orbit." (36:39-40)

Hijri Calendar is Astronomy Issue
A respected religious scholar,
Javed Ghamidi says:
"Whether a calendar is solar or lunar, it is not religious matter; it comes under Ilm-ul-Falakiat (Astronomy). "To determine the beginning of a lunar month it is not necessary to see the moon with naked eye. Ulamaa' thought it was necessary to see the moon to start a month because of not understanding the context of the Hadith. The Hadith starts with the words, "the month is 29 days......." This phrase of Hadith is not mentioned in all the discussions about this. If sighting was necessary, then why it is not required to see on the 30th of the month. If for a 30 day month, science is the basis, then why science cannot be the basis for 29 days. It is Allah who has given the knowledge of science to man. To Iftaar a fast no one watches the sun; every one relies on the watch. At suhoor time, no one watches white light appearing in the sky; every one relies on watch. In the matters of astronomy the religious scholars should not interfere. If we do not reap the benefit of science then we will become a laughing stalk in the world. The witness or Shahadah for moon sighting is not a religious matter. Instead of insisting on sighting by bare eyes or relying on mistaken witness sometimes, the benefit of scientific knowledge must be used."