Date Addition Calculator
Add years, months, weeks and days to a date, or subtract them, and see the resulting date with its weekday. A second mode counts business days (Monday to Friday), which is useful for deadlines, delivery estimates and notice periods.
Today by default.
Result
Enter a start date and an amount of time to add or subtract.
The calculation runs in this browser tab. Nothing you enter is uploaded, saved or logged.
How does adding time to a date work?
Adding days is simple arithmetic on the calendar: 30 days after 15 January is 14 February. Adding months and years is less obvious, because months differ in length. One month after 31 January cannot be 31 February, so most systems return the last day of the shorter month, 29 February in 2024. Spreadsheets do the same in their month-adding functions.
When several units are given, the order matters. This calculator applies years and months first, clamping to the end of the month where needed, and then adds the weeks and days. For subtraction the same steps are reversed in sign. Business days follow a different rule: they skip Saturdays and Sundays, so 10 business days after a Monday is two weeks later.
How to use the Date Addition Calculator
- Enter the start date. It is today by default.
- Choose whether to add or subtract, and whether to count in calendar units or in business days.
- Enter the number of years, months, weeks and days, or the number of business days. Empty fields count as zero.
- Read the resulting date and its weekday, then the day of the year and the ISO week number. Use Copy result to copy the date.
Formula
- Years and months
- date + (12 × years + months) months, keeping the day or using the last day of a shorter month
- Weeks and days
- result = that date + (7 × weeks + days) days
- Business days
- move one calendar day at a time, counting only Monday to Friday
- ISO week
- week 1 is the week that contains the first Thursday of the year
Adding one month to 31 January gives 29 February in a leap year and 28 February otherwise. Adding two months gives 31 March: every result is computed from the original date, not from the previous month-end.
Starting on a weekend, business day 1 is the next Monday. Subtracting business days from a weekend goes back to the previous Friday.
Worked example
One month after 31 January 2024. There is no 31 February, so the result is the last day of the month. These are the results the calculator gives:
| Question | Result |
|---|---|
| Input: Start date | 2024-01-31 |
| Input: Operation | Add to the date |
| Input: Count in | Years, months, weeks and days |
| Input: Months | 1 |
| Wednesday, 31 January 2024 + 1 month | Thursday, 29 February 2024 |
| Date as YYYY-MM-DD | 2024-02-29 |
| Days from the start date | +29 |
| Day of the year | 60 of 366 |
| ISO week | Week 9 of 2024 |
| Leap year | Yes |
Assumptions and limits
- Business days skip Saturdays and Sundays only. Public holidays and company closures are not skipped, since they vary by country and year. Add extra days yourself for those.
- Results must fall between the years 1 and 9999; otherwise the calculator explains that the date is out of range.
- Amounts are limited to whole numbers: up to 9,999 years, 120,000 months, 520,000 weeks, 3,652,425 days or 100,000 business days.
- The Gregorian calendar is used for all years, including those before it was adopted.
Frequently asked questions
How do I add days to a date?
Count forward day by day across months and years. 30 days after 15 January 2024 is 14 February 2024, because January has 31 days and 2024 is a leap year. The calculator does this exactly.
What is one month after 31 January?
The last day of February: 29 February in a leap year and 28 February otherwise, because there is no 31 February. This is the same rule as the month-adding functions of spreadsheets.
How do I calculate a date 90 days from now?
Leave the start date as today, choose “Add to the date” and enter 90 in the days field. The result shows the date, its weekday and its ISO week.
How do business days work?
They count Monday to Friday only. 5 business days after Monday 1 January is Monday 8 January. Public holidays are not skipped, so add days for those separately.
How do I subtract time from a date?
Choose “Subtract from the date”. For example 1 year before 1 January 2025 is 1 January 2024, and 1 day before 1 March 2024 is 29 February 2024.
What are the day of the year and the ISO week?
The day of the year runs from 1 to 365 or 366. The ISO week is the week number defined by ISO 8601: weeks start on Monday, and week 1 contains the first Thursday. Dates in early January can belong to the last week of the previous year.