Date Calculator
Add or subtract from a date, or find the days between two dates.
The date to count from.
Result date
- ISO date
- 2026-08-22
Years and months follow the calendar: the same date next year, the same day-of-month next month, with month-end clamping (adding a month to Jan 31 lands on the last day of February). Not days divided by 365.25.
How the date calculator works
This tool does two related jobs. In add or subtract mode it shifts a date by a number of whole calendar units and returns the new date. In difference mode it measures the span between two dates as calendar days, whole weeks, and business days.
All of the math runs on a vetted date library that respects the real calendar, never on a fixed-length average year or month. That matters because calendar units are not constant: months run from 28 to 31 days and leap years add a day every four years. Adding “one month” means landing on the same day number next month, and adding “one year” means the same date next year, with one important rule for the edges.
When the target day does not exist, the result clamps to the last valid day of that month. Adding a month to January 31 gives February 28 in a common year (or February 29 in a leap year), because February has no 31st. Weeks and days are simpler: a week is always seven days and a day is always one day, so those shifts are exact.
The difference mode counts the exact number of calendar days between the two dates, then splits that into whole weeks plus a remainder. The business-day count walks the days from the earlier date up to and including the later date, skipping the start date itself, and drops every Saturday and Sunday.
Worked example
Take Monday, March 9, 2026 and add 6 weeks. Six weeks is 42 days, so the result is Monday, April 20, 2026. No clamping is needed because week shifts never overflow a month.
Now measure the difference between Monday, March 9, 2026 and Friday, March 20, 2026:
- Calendar days: 11. From the 9th to the 20th is eleven days.
- Weeks: 1 week plus 4 extra days, since 11 = 7 + 4.
- Business days: 9. The counted window is March 10 through March 20, which contains exactly one weekend (Saturday the 14th and Sunday the 15th). Removing those two weekend days from the eleven leaves nine working days.
If instead you subtract 1 month from March 9, 2026 you get February 9, 2026, and subtracting 1 year gives March 9, 2025.
How to use it
- Choose a mode: add or subtract from a single date, or find the difference between two dates.
- In add or subtract mode, pick the starting date, set the direction, then enter whole numbers for years, months, weeks, and days. You can combine units, and leaving a field at zero means it has no effect.
- In difference mode, pick the two dates in either order. The result updates as you type.
- Read the ISO date in the result if you need a machine-readable value to paste elsewhere.
Limitations
This is a calendar tool, not a scheduling or legal-deadline tool. The business-day count removes only weekends, not public holidays, which differ by location and year, so confirm any real deadline against your local holiday calendar. The tool also works in whole calendar days and ignores the time of day, daylight-saving changes, and time zones. For most planning, anniversaries, and “how many days until” questions that is exactly what you want, but for contracts or anything where the exact cutoff matters, use it as a guide and verify the final date with the authority that sets the rule.
Frequently asked questions
Why not just multiply years by 365 or 365.25?
Because a real calendar does not work that way. Years vary in length (leap years have 366 days), months range from 28 to 31 days, and a fixed average like 365.25 drifts off the actual date as soon as a leap year or an odd month length gets in the way. This tool adds whole calendar units the way a calendar does: one year later is the same date next year, one month later is the same day next month. The day counts it reports are exact calendar days, not an average.
What happens when I add a month to the 31st?
It clamps to the last valid day of the target month. Adding one month to January 31 lands on February 28 in a common year, or February 29 in a leap year, because those months have no 31st. The same rule applies to adding a year to February 29: in a common year it clamps to February 28. This matches how banks and date libraries handle month-end anniversaries.
How are business days counted?
Business days are the weekdays, Monday through Friday, from the earlier date up to and including the later date. The count starts the day after the earlier date, so the start date itself is not counted, and Saturdays and Sundays are excluded. For example, Monday to the following Friday spans eleven calendar days but nine business days, because one Saturday and one Sunday fall in between.
Does the business-day count skip public holidays?
No. It only removes Saturdays and Sundays. Public holidays vary by country, region, and year, so they are not subtracted here. If your deadline depends on holidays, treat the business-day figure as an upper bound and check it against your local holiday calendar.
Does the order of the two dates matter?
No. The difference mode is symmetric: it measures the gap between the two dates regardless of which one you enter first, so the days, weeks, and business days are always reported as non-negative numbers. If you enter the same date twice, every count is zero.
Can I subtract instead of add?
Yes. Switch the direction to subtract and the same years, months, weeks, and days are taken off the starting date instead of added. You can mix units in one calculation, for example subtract one year and six months at once.