They (Ahl AL- Kitab surmised that they -the young men-) remained dormant

﴿وَلَبِثُواْ فِي كَهۡفِهِمۡ ثَلَٰثَ مِاْئَةٖ سِنِينَ وَٱزۡدَادُواْ تِسۡعٗا ﴾
[الكَهف: 25]

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

﴿ولبثوا في كهفهم ثلاث مائة سنين وازدادوا تسعا﴾ [الكَهف: 25]

Dr Kamal Omar
And they stayed in their Cave three hundred (solar) years, and added nine (for lunar years)

Dr Laleh Bakhtiar
And they lingered in expectation in their cave three hundred years, and they added nine

Dr Munir Munshey
They stayed in that cave for three centuries, and nine more years

Edward Henry Palmer
They tarried in their cave three hundred years and nine more

Farook Malik
Some say they stayed in their cave three hundred years and some add another nine

George Sale
And they remained in their cave three hundred years, and nine years over

Maududi
They remained in the Cave for three hundred years; and others added nine more years