Augustus
…
It doesn't.
حَتَّىٰٓ إِذَا بَلَغَ مَغۡرِبَ ٱلشَّمۡسِ وَجَدَهَا تَغۡرُبُ فِى عَيۡنٍ حَمِئَةٍ۬ (read from here)
(Quran 18 86)
The rough translation:
Till when he reached the مَغۡرِبَ ٱلشَّمۡسِ, he found (his vision) it set in a muddy spring,..
مَغۡرِبَ ٱلشَّمۡسِ = magriba syamsa
magrib = the time of the sunset in the west.
In this verse it implies Dzul Qarnain went to the west and he reached the place at the magrib (sun set) time.
The problem with this is that Dhul is certainly Alexander the Great, and that this section of al-Kahf is a response/reworking/discussion on the Syriac Neshana of Alexander, which is an iteration of the Alexander Romance that has a long history in the Abrahamic tradition.
Alexander in his neshana went to a place where the sun set 'near to' fetid water (and did everything else that Dhul did)
All 3 stories in al-Kahf actually relate to pre-existing traditions (although are not necessarily 'copies' of them, more reworkings)