They remained in their cave for three hundred years, and increased nine.
Translation endnotes
[18:9-27] These 19 verses contain numerous key words that strongly imply that the story of the cave have double meaning like the verses in chapter 74, and the second meaning is about a prophetic event. I have already observed examples of a hidden mathematical design, and God the most Wise will unveil it when the right time comes. When the secrets contained in the cave is revealed by divine permission, it will prove once again the truth in the assertion at 18:27. [18:25] Through a unique expression, 300+9, God provides us with the period of their stay in the cave in both the solar and lunar calendars. Three hundred solar years corresponds to three hundred and nine lunar years. See 15:87 and 72:27.