Date Calculator
Add or subtract time from a date, or measure the exact distance between two dates, including the day of the week.
Accepts YYYY-MM-DD or M/D/YYYY. Dates are sorted automatically, and the timeline shows the day gap between each one.
How the date calculator works
Adding or subtracting time. Years and months are applied first to the calendar. When a target month is shorter than the start day (for example, January 31 plus 1 month), the result is clamped to the last valid day of that month (February 28 or 29). Weeks and days are then added as an exact count of 24 hour periods.
Difference between two dates. The headline is the total number of whole days between the two dates. The same span is then converted into years, months, weeks, days, hours and minutes, so you can read the whole duration in whichever unit suits your work. The order of the two dates does not matter.
Timeline. Enter several dates and the timeline tab lays them out in order with the day gap between each one, plus the running day count from the first date. It is handy for mapping a sequence of events and seeing the durations between them at a glance.
All arithmetic is performed in Coordinated Universal Time (UTC), so daylight saving changes never shift the day of the week or the day count.
Worked examples
These follow the same rules the calculator uses. Each resulting date is shown with its day of the week.
Add 90 days to a date
Start on Thursday, January 15, 2026 and add 90 days. Ninety days is just an exact count of 24 hour periods, so there is no month clamping to worry about. The result is Wednesday, April 15, 2026. A quick check: 90 days is 12 weeks and 6 days (12 times 7, plus 6).
Days until a future date
To see how long until a date, use Difference mode. From January 1, 2026 to December 25, 2026 is 358 days, which the grid also reads as about 51.14 weeks and about 11.76 months. That December 25 falls on a Friday. Because the count is the number of whole days between the two dates, the start day itself is not counted.
Adding a month to the 31st
February has no 31st, so adding one month to January 31, 2026 cannot land on February 31. The calculator clamps to the last valid day of the target month, giving February 28, 2026 (or February 29 in a leap year). Years and months are applied before any weeks and days you also enter.
How leap years affect the count
A leap year adds February 29, making the year 366 days instead of 365. Because the calculator works from the real calendar, any span that crosses a February 29 already includes that extra day. You do not need to adjust anything.
The Gregorian rule is: a year is a leap year if it is divisible by 4, except years divisible by 100, unless they are also divisible by 400. So 2024 and 2000 are leap years, while 1900 and 2100 are not. Across a full 400 year cycle there are 97 leap years and 303 ordinary years, which makes the average calendar year 365.2425 days long.
The unit conversions in Difference mode use a rounder average of 365.25 days per year (and 365.25 divided by 12 for an average month). The whole day total in the headline is always the exact figure; the year and month conversions are averages for quick reading.
Business days versus calendar days
This calculator counts calendar days: every day of the week, weekends and public holidays alike. That is the right measure for total elapsed time, anniversaries, and most contract clauses that simply say "days".
Business days (also called working days) are different. They normally exclude Saturdays and Sundays, and often a list of public holidays too. Spreadsheet functions such as NETWORKDAYS use this convention, and unlike a plain calendar count they include both the start and end date in the tally.
If a deadline is written in business days, do not read the calendar day total here as the same number. A 7 calendar day span is usually 5 business days, but the exact figure depends on which weekends and holidays it crosses.
Common ways people use it
- Deadlines and notice periods. Add a fixed number of days to a start date to find a filing date or the end of a 30, 60, or 90 day window.
- Project and shipping timelines. Add weeks or months to a kickoff date to set milestones, or use Difference to see how many days are left until a launch.
- Anniversaries and milestones. Count the exact days between two dates to mark a 100 day milestone or a work anniversary.
- Pregnancy due dates. A common estimate adds 280 days (40 weeks) to the first day of the last menstrual period; for example, 280 days after January 1, 2026 is Thursday, October 8, 2026. This is a general estimate only, not medical advice.
- Age and elapsed time. Use Difference to read a span in years, months, weeks, days, hours, or minutes, whichever unit fits.
Frequently asked questions
Does it count both the start and end day?
No. The Difference result is the number of whole days between the two dates, so the start day is not counted. One Monday to the next Monday is 7 days, not 8. If you need both ends included, as some business day rules do, add one to the total.
Why are the months and years shown as decimals?
The headline total is exact whole days. The year, month, and week figures are that same span converted using average unit lengths (365.25 days per year, and about 30.44 days per month). Calendar months vary from 28 to 31 days, so an average is the only single number that works across any span.
Does the order of the two dates matter?
No. Difference mode always measures the absolute distance between the dates, so swapping From and To gives the same answer.
Do time zones or daylight saving change the result?
No. All arithmetic runs in Coordinated Universal Time (UTC), so daylight saving shifts never move the day of the week or the day count.
Does this count business days?
No. It counts calendar days, including weekends and holidays. See the section above for the difference.