How to use the D-day & Date Calculator
- Pick a start date and an end date. The form opens filled with today and the date 100 days from now.
- Choose whether the start date counts as day 1. That is the usual way to count anniversaries such as "day 100".
- See the duration, weekdays, the date of day N, milestone dates for days 100, 200, 300, 365, 500 and 1000, and a countdown from today.
How it works — D-day & Date Calculator
The gap between two dates is found by turning each date into a whole-day number and subtracting. To avoid time zone and daylight saving effects, every date is calculated at midnight UTC. When the start date counts as day 1, one day is added to the duration, and day N falls on the start date plus N−1 days. For example, if January 1 is day 1, day 100 is April 10 in a non-leap year. The calendar span subtracts years, months and days in that order and borrows the length of the previous month when days run short. Weekdays are counted from the start to the end date excluding Saturdays and Sundays; public holidays are not removed.
When this helps
Use it for a probation end date, the last working day after a notice period, a contract expiry, days left before an exam, a baby's 100th day, or any anniversary where the gap between two dates matters. If the end date comes before the start date, the tool swaps them and tells you so.
Tips
- For a three-month probation, count the start as day 1 and look up the date of day 90.
- If a contract says "within N days", check whether the contract counts the first day.
- Weekday counts are a rough guide to working days; public holidays are not excluded.
- The countdown uses the date on which you opened this page.
Notes & limitations
Legal deadline rules (employment, tenancy, limitation periods and similar) may exclude the first day or follow other rules, so results can differ from this tool. Confirm important dates against the contract, the relevant law or a professional.
FAQ
What changes if I count the start date?
The duration grows by one day and the date of day N moves one day earlier. Use it when the first day counts as day 1.
Are public holidays excluded from weekdays?
No. Only Monday to Friday are counted and holidays are not removed.
Does it handle leap years?
Yes. Dates are calculated on the real calendar, so February 29 is handled correctly.