noun
12th day of the month
Refers specifically to the 12th day of a month, not a duration.
会議は１２日に行われます。
The meeting will be held on the 12th.
noun
12 days
Refers to a duration of twelve days.
この薬は１２日間飲み続けてください。
Please continue taking this medicine for 12 days.
Compound of 十二 (twelve) and 日 (day).