How to use the add days to date
- Enter your start date in YYYY-MM-DD format.
- Type the amount to add (use a negative number to subtract).
- Choose the unit: days, weeks, months or years.
- Check Business days only if you need to skip Saturdays and Sundays (applies to days).
- Read the result date, weekday and ISO week number below.
How date addition works
When you add days to date, the tool converts the input into a UTC timestamp using Date.UTC(year, monthIndex, day). Working in UTC avoids daylight-saving-time shifts that could otherwise move the result by an hour and change the calendar day.
For days and weeks, the math is simple: multiply the amount by 86,400,000 milliseconds (or by 7 for weeks) and add it to the timestamp. For example, adding 30 days to 2026-01-15 means advancing 30 x 86,400,000 ms, landing on 2026-02-14.
For months and years, the tool adds directly to the month or year component of the date. If the resulting day exceeds the number of days in the target month, it clamps to the last valid day. Adding 1 month to January 31 yields February 28 in a common year or February 29 in a leap year.
The amount must be a whole number and cannot be blank. To keep results inside the calendar the tool accepts up to 3,000,000 days, 400,000 weeks, 100,000 months or 8,000 years in either direction, and it reports an error if the result would fall outside years 1 to 9999.
Business days vs calendar days
A calendar day counts every single day including weekends. A business day skips Saturdays and Sundays. When you enable the business-days option, the tool steps forward one day at a time, ignoring weekend dates until it has counted the requested number of weekdays.
Adding 10 business days to Thursday, 2026-01-15 lands on Thursday, 2026-01-29. The two intervening weekends (Jan 17-18 and Jan 24-25) are skipped, so 14 calendar days pass instead of 10. This is essential for project deadlines, shipping estimates and contract terms that specify working days.
Negative amounts work the same way in reverse: subtracting 5 business days from Monday, 2026-01-19 lands on Monday, 2026-01-12, skipping the weekend of Jan 17-18.
Common date addition examples
The table below shows several typical scenarios when you use this date plus days calculator:
| Start date | Add | Result | Note |
|---|---|---|---|
| 2026-01-15 | 30 days | 2026-02-14 | Standard calendar addition |
| 2026-01-15 | 10 business days | 2026-01-29 | Skips 2 weekends |
| 2026-01-31 | 1 month | 2026-02-28 | Clamped to end of Feb |
| 2024-02-29 | 1 year | 2025-02-28 | Leap day clamped |
| 2026-03-01 | -14 days | 2026-02-15 | Subtraction crosses month |
| 2026-06-15 | 2 weeks | 2026-06-29 | Exactly 14 calendar days |
The month-end clamp rule
Adding months or years can produce an invalid date like February 30 or April 31. The standard solution, used by most financial and legal systems, is the month-end clamp rule: if the target day exceeds the last day of the resulting month, the date snaps to that last day.
For instance, March 31 plus 1 month would nominally be April 31, which does not exist. The tool returns April 30. Similarly, August 31 plus 6 months targets February 31, which clamps to February 28 (or 29 in a leap year).
This behaviour matches the logic in spreadsheet functions like EDATE and is important for recurring billing cycles, subscription renewals and maturity dates on financial instruments.
ISO week numbers explained
The result includes the ISO 8601 week number. Under this standard, weeks start on Monday and the first week of the year is the one containing the first Thursday. This means January 1 can fall in week 52 or 53 of the previous year if it is a Friday, Saturday or Sunday.
The ISO week number is widely used in European business planning, payroll systems and logistics. If you need to find the span of a specific week, you can pair this tool with a week number calculator. For measuring the gap between two dates rather than adding to one, see the days between dates calculator.
Worked examples
| Example | Input | Result |
|---|---|---|
| 30 calendar days from mid-January | Start date: 2026-01-15; Amount to add: 30; Unit: days; Business days only (skip weekends): no | Result date: 2026-02-14; Weekday: Saturday; ISO week: 2026-W07 |
| 90 business days forward | Start date: 2026-01-15; Amount to add: 90; Unit: days; Business days only (skip weekends): yes | Result date: 2026-05-21; Weekday: Thursday; ISO week: 2026-W21 |
| Subtract 3 months | Start date: 2026-05-31; Amount to add: -3; Unit: months; Business days only (skip weekends): no | Result date: 2026-02-28; Weekday: Saturday; ISO week: 2026-W09 |
| Add 2 years from leap day | Start date: 2024-02-29; Amount to add: 2; Unit: years; Business days only (skip weekends): no | Result date: 2026-02-28; Weekday: Saturday; ISO week: 2026-W09 |
FAQ
How do I subtract days instead of adding them?
Enter a negative number in the Amount field. For example, setting the amount to -14 subtracts two weeks from the start date.
What happens when I add a month to January 31?
The tool uses the month-end clamp rule. Since February never has 31 days, the result is clamped to February 28 in a common year or February 29 in a leap year.
Do business days include public holidays?
No. The business-days option only skips Saturdays and Sundays. Public holidays vary by country and are not accounted for in this calculation.
Why does the tool use UTC for date math?
Using UTC prevents daylight-saving-time transitions from shifting the result by an hour, which could cause the calculated date to land on the wrong calendar day.
Can I add weeks, months or years instead of days?
Yes. Select the desired unit from the dropdown. Weeks are converted to 7 calendar days each, while months and years use calendar arithmetic with end-of-month clamping.
Is my date sent to a server?
No. All calculations happen in your browser. Nothing is uploaded or stored.
What date format should I use?
Enter dates in YYYY-MM-DD format, such as 2026-01-15. This is the ISO 8601 standard and avoids ambiguity between month-first and day-first conventions.