noun
30th day of the month
Refers specifically to the 30th calendar day; used in dates like ３月３０日 (March 30th).
会議は３月３０日に行われます。
The meeting will be held on March 30th.
３０日までに書類を提出してください。
Please submit the documents by the 30th.
noun
30 days
Indicates a duration of thirty days; often used with counters or in expressions like ３０日間.
この薬は３０日分あります。
This medicine is a 30-day supply.
３０日間、毎日運動を続けた。
I exercised every day for 30 days.