the answers is wrong, there's no difference between يوم and نهار .. in Quran " سبع ليال وثمانية أيام " and " ومن آناء الليل فسبّح وأطراف النهار " .. as you can see the opposite to them is " ليل " and both of them is used to express the "day" from dawn to sunset.

يوم = day the 24 hours نهار = it means a part of the day it starts at morning and ends at sunset

يوم = day (the 24 hours) نهار = day (when the sun is up in the sky, the opposite is لَيْل = night)

If the words يوم and نهار have one meaning, it means that one of them is useless, nowadays, we use the word يوم in an astronomical sense, wich means 24 hours (and it's legal) . on the other hand; نهار means the period between sunrise and sunset. If I said "سألتقي بك يوم الجمعة" ", it means at any hour on Friday, daylight or night. but if I sadi " سألتقي بك نهار الجمعة" it means only in daylight. Even though, you can use يوم in the place of نهار, but the opposite is incorrect. it's not correct that يوم and نهار have the same meaning, and it's tricky, so don't confuse yourself.

يوم means all 24-hours. نهار means the part of the day that start with sunrise and ends with sunset.