Days and weeks
result = start date ± n days (weeks: n × 7 days)
Add or subtract time from a date
Find the date reached by adding or subtracting days, weeks, months or years, with a clear rule for month ends and leap years.
Enter a valid date and a whole, non-negative amount.
Resulting date
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How does this tool work?
This tool answers questions such as “what date is 90 days from today?”, “what date falls 6 months after 31 August?” or “what day was it 2 years ago?”. Pick a start date, add or subtract a number of days, weeks, months or years, and read the resulting date with its day of the week. The calculation is instant and runs entirely in your browser.
Days and weeks are fixed lengths: the tool moves forward or back by the exact number of days (one week = 7 days), crossing month ends, year ends and 29 February without approximation.
Months and years have no fixed length: the tool keeps the same day of the month and changes the month or year. If that day does not exist in the target month, it uses the last day of the month: 31 January + 1 month = 28 February (29 in a leap year), 31 March − 1 month = 28 or 29 February, 29 February 2024 + 1 year = 28 February 2025. This is the default (“constrain”) behaviour of the JavaScript Temporal standard and of most spreadsheets.
In practice, adding “1 month” is not the same as adding “30 days”, and adding months is not always reversible (31 January + 1 month − 1 month = 28 January). Choose the unit that matches the rule you are applying.
How to use this tool
Choose the start date
Type the date or pick it from the calendar.
Set the operation and amount
Choose Add or Subtract, enter a whole number and the unit: days, weeks, months or years.
Read the resulting date
The result shows the day of the week; copy the calculation link to keep or share it.
Calculation rules
Two different rules depending on the unit.
Months and years
result = (year, month ± n, min(day, days in target month)); 1 year = 12 months
- n
- amount entered, a whole number of zero or more
- min(…)
- the smaller of the two numbers: the day is moved back to the last day of the month if it does not exist
Use cases
“In X days” deadlines
Find the date of a 30-day reminder, a 45-day payment term or a 90-day warranty from a start date.
Monthly or yearly deadlines
Work out the end of a 12-month subscription, a 3-month notice period or a contract anniversary.
Project planning
Place a milestone 6 weeks after a kick-off or count back 8 weeks from a delivery date.
Past dates
Find the date 100 days, 18 months or 10 years ago, for example for a reference period.
Examples
100 days after 11 October 2026
20 days to finish October, then 30 (November) and 31 (December): 19 days remain in January.
Input
10/11/2026 + 100 daysResult
Tuesday, January 19, 2027One month after 31 January
There is no 31 February, so the date moves back to the last day of February.
Input
01/31/2026 + 1 monthResult
Saturday, February 28, 2026One year after 29 February
2025 is not a leap year; adding 4 years lands on 29 February 2028 again.
Input
02/29/2024 + 1 yearResult
Friday, February 28, 2025Tips and best practices
- For a deadline in business days (excluding weekends), use the business days calculator, which shifts a date by a number of weekdays.
- To combine units (2 months and 10 days), run two calculations; add the months first, then the days, as the order can change the result at the end of a month.
- Legal deadlines are often counted from the day after the event and may roll over when the last day is a Saturday, Sunday or public holiday: check the applicable rule before relying on a date.
- To measure the gap between two known dates, use the date difference calculator instead.
Sources and references
- Claus Tøndering, The Gregorian Calendar (opens in a new tab), Calendar FAQPage in English
- ISO — ISO 8601 date and time format (opens in a new tab)Page in English
- TC39, Temporal.PlainDate (opens in a new tab), Temporal documentationPage in English
Frequently asked questions
What date is 30, 60 or 90 days from today?
Keep today as the start date, choose “Add”, enter 30, 60 or 90 and the unit Days. The result accounts for the real length of each month and for leap years.
Is adding 1 month the same as adding 30 days?
No. “1 month” keeps the day of the month (15 March to 15 April), whereas “30 days” moves exactly 30 days (15 March to 14 April). Use the unit stated in your contract or rules.
What happens to the 31st when the target month has 30 days?
The date moves back to the last day of the month: 31 May + 1 month = 30 June, 31 January + 1 month = 28 or 29 February. The day never spills over into the next month.
Why does 31 January + 1 month − 1 month give 28 January?
Because the first step moves the date to 28 February; subtracting a month then starts from the 28th and lands on 28 January. Adding months is not always reversible; that is inherent to the calendar.
Can I add hours or minutes?
No, the tool works to the day. To add hours, use the time duration calculator or the hours and minutes sum tool.
Does it work for very old dates?
Yes, it applies the Gregorian calendar to every date, including before its adoption in 1582 (proleptic Gregorian calendar). Convert historical dates recorded in the Julian calendar first.
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