Some say, The Refugees in the Cave spend (solar) years ([lunar] year and 9 months) in the cave
Faridul Haque
And they stayed in their Cave for three hundred years * and nine more *. (* 300 according to the Solar calendar and 309 according to the Lunar calendar.
Hasan Al-Fatih Qaribullah
And they stayed in the Cave three hundred years and to that they added nine more
(And some say) they remained in their cave for three hundred years and added nine to it.
Sayyed Abbas Sadr-Ameli
And they remained in their Cave three hundred years, and (to that also) they added nine more
Al-muntakhab fi tafsir al-Qur'an al-Karim
They (Ahl AL- Kitab surmised that they -the young men-) remained dormant in the Cave for three hundred years -solar calendar- with an additional nine years -lunar calendar-