noun
18th day of the month
Refers specifically to the 18th day of a month, not a duration.
会議は１８日に行われます。
The meeting will be held on the 18th.
noun
18 days
Indicates a period of 18 days. Often used with counters or in contexts like 'for 18 days'.
この仕事は１８日かかりました。
This work took 18 days.
Explicitly means 'a period of 18 days', while １８日 can mean either the date or the duration depending on context.
Compound of 十八 (じゅうはち, 'eighteen') and 日 (にち, 'day'). The reading にち is the standard on-yomi for 日 in date expressions.