As-Salamu 'Alaykum: If you are indeed able to forward this query about the duration of time mentioned in the ayat of Suratul-kahf to a knowledgeable scholar of the Arabic language and Qur'an so that the time period in the ayah:

99 ۞ وَتَرَكْنَا بَعْضَهُمْ يَوْمَئِذٍ يَمُوجُ فِي بَعْضٍ ۖ وَنُفِخَ فِي الصُّورِ فَجَمَعْنَاهُمْ جَمْعًا 100وَعَرَضْنَا جَهَنَّمَ يَوْمَئِذٍ لِّلْكَافِرِينَ عَرْضًا ( سورة الكهف , Al-Kahf, Chapter #18, Verse #99-100)

Can be explained. "yawmaithin"="On that day", yet the word 'yawm' has the potential to mean 1000 years or 50,000 years as mentioned in the Qur'an. The duration of time is still not yet known. Is it then better to translate "yawmaithin" as "In that period of time"? As-Salamu 'Alaykum.

Alhamdu Lillah

The word يومئذ   does not indicate any duration or a period of time. Simply, it means: on that day.

The fact that one day in the Hereafter is as long as 1000 years or 50000 years of the worldly life has nothing to do with the above argument. They are two different concepts.