１８じゅうはち日にち

noun

18th day of the month

Refers specifically to the 18th day of a month, not a duration.

• 会かい議ぎは１８日にちに行おこなわれます。

  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'.

• この仕し事ごとは１８日にちかかりました。

  This work took 18 days.