Translation:A few days
며칠 is not attested to be from 몇 일, in case anyone was wondering. The temptation to write out 몇 일, though, is pretty strong given 몇 초, 몇 분, and 몇 시, but it is officially considered incorrect. Perhaps someone bright will figure out the true etymology of the term one day and show us the morphemes that make up the word. Maybe it’s an older Chinese borrowing that became part of a fixed expression?
ㅡ → ㅣ is actually an attested sound change. Thats how 김 came from 금.
If 몇흘 is the earlier form, the modern word should be written as 몇힐 to keep with morphological principles.