Al-Kahf 18:25

<h2>The Length of their Stay in the Cave</h2><p>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.'</p><div class="arabic uthmani">قُلِ اللَّهُ أَعْلَمُ بِمَا لَبِثُواْ</div><p>(Say: "Allah knows best how long they stayed...") `If you are asked about how long they stayed, and you have no knowledge of that and no revelation from Allah about it, then do not say anything. Rather say something like this:</p><div class="arabic uthmani">اللَّهُ أَعْلَمُ بِمَا لَبِثُواْ لَهُ غَيْبُ السَّمَـوَتِ وَالاٌّرْضِ</div><p>(Allah knows best how long they stayed. With Him is (the knowledge of) the Unseen of the heavens and the earth.)" meaning, no one knows about that except Him, and whoever among His creatures He chooses to tell. What we have said here is the view of more …

And (it is said) they tarried in their Cave three hundred years and add nine.

So they stayed in their Cave three hundred years, and (some) add nine (more)

And they stayed in their Cave three hundred (solar) years, and add nine (for lunar years).

And they stayed in their cave for three hundred years, adding nine.