What Is a Calendar Day?
Definition
Calendar Day: A date on the calendar: a full day from midnight to midnight. Weekends and public holidays are calendar days too; a week has 7 calendar days.
A calendar day is a full day on the calendar, from midnight to midnight, that matches one date. Weekends and public holidays are calendar days too, so a week has 7 calendar days but usually 5 business days. The date calculators do not use hours; they count each date as one calendar day.
Where it appears in the formulas
days = end − start: the start day counts and the end day does not. 2026-09-14 to 2026-10-14 is 30 calendar days, 22 of them business days. By the day counter, there are 110 calendar days from 2026-09-13 to 2027-01-01.
Dates, not hours
Because the count uses dates, moving the clocks forward or back never changes the number of days. A contract or a law may set a period in business days or leave out the first day; for a deadline, follow the rule of that text.