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, December 21, 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.
Frequently asked questions
Does the countdown count today?
The number here is the target date minus today, nothing more. If the target is today it reads D-DAY, tomorrow is D-1 and yesterday is D+1. In other words today is not counted as one of the days. If your convention counts today as day one, add 1 to the number shown.
How do I work out the 100th or 1000th day?
Use the date calculation tab and add the number of days to a base date. Note that in the Korean anniversary convention the first day counts as day one, so the 100th day falls 99 days later: enter 99, not 100. The 1000th day works out the same way at 999.
I want the last day included in the day count
Tick the include both ends option under the date difference tab and the total goes up by one. Use it for event periods or hotel stays where the first day counts as a full day. The same panel also shows the gap in weeks and days and in years, months and days, which helps when restating a period in another unit.
Are public holidays excluded from the working days?
The working days tab excludes Saturdays and Sundays only. Public holidays and company closures are not taken into account, so subtract the holidays that fall in the range yourself. Both the start and end dates are counted, so a single weekday entered twice comes out as one day.
How do I check an exact age in years?
Put the birth date and today into the date difference tab and it breaks the gap into years, months and days: the years figure is the exact age. In a year where the birthday has not happened yet it already comes out one lower, so no extra adjustment is needed. Anniversaries work the same way.
What happens adding a month to a month end?
When the target month has no matching day, one month after 31 January for example, the result snaps to the last day of that month: 28 February, or 29 in a leap year. This applies to the month and year units, while days and weeks simply shift by that many days.