Lunar Year:

• The Islamic Calendar consists of 12 Lunar Months. 
• And every month consists of one circle of the moon around planet Earth.
• The moon takes 29 to 30 days to complete one circle around the Earth. 
• Thus, Islamic Lunar Year consists of 355 days (in which the moon revolves around the Earth 12 times)

Solar Year:

• Solar year means one circle by planet Earth around the sun. 
• Earth takes 365 days to complete this one circle. 
• A solar year has nothing to do with the system of 12 months, 4 weeks in a month, or 7 days in a week. This system of 12 months + 4 weeks + 7 days was created by the people who earlier followed the lunar year. Later, when they started following the solar year, they introduced this system into the solar year too for their convenience as they were used to it. 
  
• Instead of 12 months, the solar year is mainly divided into 4 seasons of spring, summer, autumn and winter, which repeat at the exact same intervals in the solar years. 

All the changes that take place on the planet Earth basically belong to the position of the sun, and not to the position of the moon. 

Human life is also mainly affected by the position of the sun. All the crops and businesses and seasons are linked to the position of the sun. As compared to the sun, the position of the moon plays a negligible role in human life. 

Therefore, the Islamic Lunar Calendar is faulty and deficient, and it was created by the people of the time of ignorance, who didn’t know that the earth completes its circle around the sun in 365 days. 

Since there is no All-Knowing Allah present in the heavens, and Muhammad was himself creating this religion of Islam and Quran, and he himself belonged to that culture of the time of ignorance, that is why Muhammad made this double mistake in Quran:

1. Firstly, Muhammad chose this faulty and deficient Lunar Calendar, instead of the solar calendar
   (Note: Solar Calendar was already present in the world at the time of Muhammad, but Muhammad didn’t know about it as he lived in an area where people were ignorant of the solar year)
2. And moreover, Muhammad also made this claim in Quran that these 12 months existed since the beginning of the universe. 

Quran 9:36:
Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah since the day He created the heavens and the earth; of these, four are sacred. That is the correct religion, so do not wrong yourselves during them. (Translation by Sahih International).

But how is it possible when there existed no moon and earth, no day and night, no months and year at the beginning of the universe? The earth came into being 9 billion years after the creation of the remaining universe. And the moon came into being even after planet Earth.

But obviously, Muhammad didn’t know it.

What to talk about the lunar year, but even the solar year also has nothing to do with the heavens, as the sun was also not present at the time of the creation of the universe. 

Day and Night in the whole Universe

Similarly, look at Verse 79:27-29:

أأنْتُم أَشَدُّ خَلْقاً أَمِ السَّمَآءُ بَناهَا* رَفَعَ سَمْكَهَا فَسَوَّاهَا* وَأَغْطَشَ لَيْلَهَا وَأَخْرَجَ ضُحَاهَا* وَالأرْضَ بَعْدَ ذَلِكَ دَحَاهَا* أَخْرَجَ مِنْهَا مَآءَها وَمَرْعَاهَا* وَالْجِبَالَ أَرْسَاهَا* مَتَاعاً لَّكُمْ وَلأنْعَامِكُمْ

Is it harder to create you or the heaven? But Allah built it ,and raised its vault high and proportioned it; and covered its night with darkness and brought forth from it its day.

The writer of the Quran (i.e. Muhammad) made this story while people back then believed that there is only one sun in the universe. Thus, when the sun sets, then it is not only night on the earth, but in heaven too. Similarly, when the sun rises, then it is not only a day on the earth, but in heaven too.

Muhammad thought that he was free to fabricate such stories about the heavens and no one is ever able to investigate such stories. But he didn’t know that a time will come when science will be able to investigate his stories and find out that his claims are nothing else than lies.

The start of the “Weekly System” and Islam copying it from earlier cultures

The system of 4 weeks in a month is also a flawed system. A lunar month is equal to 29.53 days and it could not be divided into 4 weeks. 

This system originated in Sumerian culture originally. 

https://www.livescience.com/45432-days-of-the-week.html
The seven-day week originates from the calendar of the Babylonians, which in turn is based on a Sumerian calendar dated to 21st-century B.C. Seven days corresponds to the time it takes for a moon to transition between each phase: full, waning half, new and waxing half. Because the moon cycle is 29.53 days long, the Babylonians would insert one or two days into the final week of each month.

Muhammad then further claimed that the universe was created on different days of the week. 

Sahih Muslim, Book of creation: