Tafsir of Al-Kahf 18:25

The Length of their Stay in the Cave Allah says:

ولبثوا في كهفهم ثﻼث ماية سنين وازدادوا تسعا

And they stayed in their cave three hundred years, adding nine. Here Allah tells His Messenger the length of time the people of the Cave spent in their cave, from the time when He caused them to sleep until the time when He resurrected them and caused the people of that era to find them. The length of time was three hundred plus nine years in lunar years, which is three hundred years in solar years. The difference between one hundred lunar years and one hundred solar years is three years, which is why after mentioning three hundred, Allah says, `adding nine.'

قل الله أعلم بما لبثوا

له غيب السماوات واﻻرض أبصر به وأسمع ما لهم من دونه من ولي وﻻ يشرك في حكمه أحدا