Date & D-day Calculator
date-calc• Count down to a date (D-day), days between two dates, add/subtract days, and business days. • See the day of the week instantly.
D-100
100 days left
Monday, November 9, 2026
What trips people up in date maths
When counting the days between two dates, the thing people disagree on is whether the start day counts. A plain subtraction leaves it out, but for an event period or a hotel stay where the first day counts as a day, you need to add one. Decide which convention you are using before you start.
The same goes for countdowns. Whether today is D-1 or D-0 depends on local habit, so for anything with a deadline it pays to confirm which convention the other side uses. When adding days, month ends and leap years make manual counting error-prone — let the calculation handle it.
Legal and contractual periods can differ again. Whether “within N days” includes the first day, or shifts when the last day falls on a holiday, depends on the specific rule. For anything that matters, check the relevant guidance once more. Dates you enter are calculated only in your browser.