1The specific date on which a planned event, activity, etc. is (or will be) held.
会議の日にちを来週の月曜に変更した。
The meeting date was changed to next Monday.
旅行の日にちがまだ決まっていない。
The date of the trip hasn't been decided yet.
打ち合わせの日にちと時間を知らせてください。
Please let me know the date and time of the meeting.
結婚式の日にちは来年の春に決めた。
We set the date of the wedding for next spring.
2The span of days something takes or the number of days remaining, typically in set phrases like 日にちがかかる (it takes days) or 日にちがある (there are still days left).
傷が治るまでには、まだ日にちがかかりそうだ。
It looks like it will still take a number of days for the wound to heal.
引っ越しまでまだ日にちがあるので、荷造りは後でいい。
There are still days until the move, so I can pack later.
完成までにどのくらい日にちがかかりますか。
About how many days will it take to finish?