And they stayed in this Place of Refuge three hundred years and extended (their stay) another nine (years). (18:25)  
Commentary
NUMBER 300: As with any good story, many times, a reader wants to know all of the juicy details, sometimes even those things which really don’t have any benefit for the reader, but still, they are intrigued and want to know those finer points.
In the story of the People of the Cave, one of final things which Allah informs us about is how long they stayed in the cave and although it is not a value-added piece of information, still Allah mentions in this verse that they resided in the cave, before their waking up, 300 years and then 9 more – so a total of 309 years before they woke up.
Why did Allah create this verse in this fashion and not just come out and say that they stayed in the cave for 309 years? 
Well, one opinion is that the 300 year figure was given in solar years and if you were to calculate the time spent in the cave in lunar years, using the lunar calendar, then it would be 309 years because on average, a year on the solar calendar has 365.24 days where as the lunar calendar, which is measured on the moon’s rotation around the earth has 354 days, as:
­365.24219-354.367056=10.875134*300=3262.5402/365.24219=8.93253926 – or rounded up to 9
In Tafsīr Majmaʿ al-Bayān it is mentioned that once, a group of Jews came to Imam ʿAlī and asked him a question in regards to the length of stay of the Companions of the Cave – as the story of the “Seven Sleepers” was well known to the Jews and Christians of that era. They were given the reply that they stayed in the cave for 309 years to which the Jewish guests corrected the Imam that it was only 300 years to this, Imam ʿAlī replied that the 9 years added is the calculation of the difference which exists between a solar year and a lunar year!
At the end of the day, it is not that important to know the exact time which was spent – rather, what is more important is to study the story, know the history behind what transpired and why it occurred and then see how we can implement this and other such historical events in our lives. 
Ultimately as Muslims we need to get over the ‘rationalization’ of our religion and trying to understand everything from the point of view of science. We must realize that sometimes, things are as they are because Allah wants to really see if we are going to unconditionally submit to His orders.
Point of Reflection
There are many unknowns in religion; and some verses of the Quran which today “may not make sense” to us – we must realize that we should never feel that we are above the Quran or Islamic teachings or that they must answer to us. Rather, we need to realize and accept the fact that we are very limited in our knowledge and there is much of the unseen world that we have yet to discover.