２１にじゅういち日にち

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Meanings

1

21st day of the month

Used to specify the 21st day of any month in dates.

会かい議ぎは２１日にちに行おこなわれます。
The meeting will be held on the 21st.

2

21 days

Indicates a duration of 21 days. Often used with counters or in expressions of time span.

この薬くすりは２１日にち間かん飲のみ続つづけてください。
Please continue taking this medicine for 21 days.