noun
number of days; days
Used to count or refer to a number of days, often in formal or written contexts. More common in everyday speech is 日数 (にっすう).
工事の完了までに要する日子は約30日です。
The number of days required to complete the construction is about 30 days.
契約書には支払い期限の日子が明記されている。
The contract specifies the number of days for the payment deadline.