Solar vs lunar.
1400 years ago the Arabs used the lunar calendar which was first invented by the Sumerians. At that time they thought that the lunar calendar was invented first. But today we know that this is false. Modern archeological discoveries show that the solar calendar was actually used since the stone age.
The Sumerians and Babylonians were probably the first people to use what we now recognise as a modern calendar. The basis of this was the lunar cycle and the Sumerian year was made up of 12 lunar cycles. But 12 moon cycles fall short in relation to the solar year. So not to fall out of sync with the seasons, the Sumerian astronomers introduced an extra month every four years... Subsequently the Sumerian calendar was not only absorbed into the Babylonian calendar, but a lot of other cultures such as the Ancient Greeks, Egyptians and Hebrews also absorbed elements into their own calendar system. In particular the Sumerian calendar was used as a blueprint of many religious calendars who are still in use today.
Living With The Moon, The Sumerian Calendar, 2019
For thousands of years the Sumerians were believed to have invented the lunar calendar first. However modern archeological discoveries show that this is not true, the solar calendar was first invented in the stone age.
A number of prehistoric structures have been proposed as having had the purpose of timekeeping (typically keeping track of the course of the solar year). This includes many megalithic structures, and reconstructed arrangements going back far into the Neolithic period.
In Victoria, Australia, a Wurdi Youang stone arrangement could date back more than 11,000 years, with some estimates dating it to be older than 20,000 years. This estimate is based on the inaccuracy of the calendar, which is consistent with how the Earth's orbit is thought to have changed during that time. The site is found near the world's oldest known site of permanent aquaculture.
A ceramic artefact from Bulgaria, known as the Slatino furnace model, has been pronounced by local archeologists and media to be the oldest known calendar representation, a claim not endorsed in mainstream views.
A mesolithic arrangement of twelve pits and an arc found in Warren Field, Aberdeenshire, Scotland, dated to roughly 10,000 years ago, has been described as a lunar calendar and was dubbed the "world's oldest known calendar" in 2013.
The Oldest European calendar is found near to Vukovar in modern-day Croatia. It is a ceramic vessel bearing inscribed ideograms of celestial objects.
Wikipedia, History Of Calendars, 2019
The solar calendar was first invented during the stone age while the lunar calendar was invented thousands of years later. We know this from modern archeological discoveries. However this was portrayed in the Quran 1400 years before it was discovered.
[Quran 18:25] And they stayed in their cave for three hundred years, and increased by nine.
This is the transition from solar calendar to lunar calendar, from 300 solar years to 309 lunar years. Hence in the Quran the solar calendar predated the lunar calendar. No mistakes in the Quran.
How could an illiterate man who lived 1400 years ago have known that the solar calendar predated the lunar calendar?