Date Difference Calculator — Days and D-day
Days between dates, a countdown timer and add/subtract days in one tool
Runs in your browser · nothing is uploaded
Enter both dates.
What it is
A date calculator for the questions people actually ask: how many days until an event, how long a contract or trip lasts, how old a project is, or what the date will be 90 days from today. It works in three ways. Between two dates gives the total days, weeks and days, calendar months and days, and the weekday of each date. Countdown tells you how many days are left until, or have passed since, a target date. Add / subtract days returns the resulting date and weekday.
How to use
- Choose a mode from the tabs: between two dates, countdown, or add / subtract days.
- For the difference, enter a start and end date. The start date is filled with today. Tick the option to count the first day as well.
- For the countdown, enter the target date and read the D-number.
- To add or subtract days, enter a base date and a number of days, then choose after or before.
- Results update instantly. Use the copy button beside the main result.
How it works
- Calendar only: dates are converted to day numbers using just year, month and day. No clock time or time zone is involved, so daylight saving changes never move the answer by a day.
- Leap years: divisible by 4 is a leap year, except years divisible by 100 unless they are also divisible by 400.
- Include start day: behaves like moving the end date one day later, and is applied to the days, weeks and months shown.
- Months and days: the calculator adds whole months to the earlier date for as long as the result does not pass the later date, then shows the remaining days. A missing day in a short month is replaced by that month’s last day.
- Reversed order: if the start is after the end, the dates are swapped and a note says so.
- Supported years are 1 to 9999.
Examples
| Task | Input | Result |
|---|---|---|
| Days | 2024-05-01 to 2024-05-10 | 9 days (10 inclusive) |
| Whole year | 2024-01-01 to 2024-12-25 | 359 days, 51 weeks 2 days, 11 months 24 days |
| Month end | 2023-01-31 to 2023-03-01 | 29 days, 1 month 1 day |
| Leap year | 2024-02-28 to 2024-03-01 | 2 days (1 day in 2023) |
| Add days | 100 days after 2024-01-01 | 2024-04-10 |
FAQ
How do you count days between two dates?
Subtract the start date from the end date. May 1 to May 10 is 9 days. If you want to count the first day too, as many contracts and trackers do, tick the include-start option to get 10 days.
What is one month after January 31?
February has no 31st, so the last day of February is used (Feb 28 in a common year and Feb 29 in a leap year). Months are calendar months, and any leftover time is shown in days.
Do daylight saving time and leap years affect the result?
No. Only the calendar year, month and day are used, never clock times or time zones, so a day with 23 or 25 hours cannot change the count. Leap days and the century rule are handled.
What does the countdown label mean?
D-10 means ten days remain until the target, D-Day is the day itself, and D+3 is three days after it. Today is taken from your device's local date.
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