Tafseer Ibn Kathir of AL HADIID [6]

يُولِجُ اللَّيْلَ فِي النَّهَارِ وَيُولِجُ النَّهَارَ فِي اللَّيْلِ وَهُوَ عَلِيمٌ بِذَاتِ الصُّدُورِ

He causes the night to enter the day and causes the day to enter the night, and he is Knowing of that within the breasts.