What this calculator does
This tool tells you the exact date you land on after adding or subtracting a number of days, weeks, months or years from any starting date. Instead of counting squares on a calendar by hand, you type the base date, pick an operation and instantly get the result along with its day of the week.
It is handy any time you deal with deadlines: the due date of a legal filing, when an invoice or contract expires, the delivery date you promised a client, the end of a 12-month warranty, the close of a 90-day trial period, or a follow-up appointment three weeks out. Everything runs in your browser, so your dates never leave your device.
How to use it
- Base date: the day you count from. It defaults to today, but you can change it.
- Operation: choose Add to move forward or Subtract to move backward.
- Amount: type the number (for example, 90).
- Unit: pick days, weeks, months or years.
The result shows up right away: the new date with its weekday, the same date in ISO format (YYYY-MM-DD) ready to copy, and the total difference in calendar days.
How it is calculated
- Days are added one at a time to the base date.
- Weeks are turned into days by multiplying by 7.
- Months and years keep the same day-of-month at the target. When that day does not exist (say, the 31st in a 30-day month), the date rolls back to the last day of that month.
Worked example
Start from January 15, 2026 (a Thursday) and add 90 days. January contributes 16 days to reach the 31st, February 2026 adds 28 (it is not a leap year), and March adds 31, putting us at 75 days by March 31. The remaining 15 days land on April 15, 2026 (Wednesday).
Interestingly, adding 3 months to the same date also lands on April 15, 2026: the 15th exists in April, so it is preserved and matches the 90-day count exactly.
Quick reference
Starting from January 15, 2026:
| Operation | Result date | Weekday |
|---|---|---|
| +7 days | January 22, 2026 | Thursday |
| +30 days | February 14, 2026 | Saturday |
| +90 days | April 15, 2026 | Wednesday |
| +6 months | July 15, 2026 | Wednesday |
| +1 year | January 15, 2027 | Friday |
| −30 days | December 16, 2025 | Tuesday |
Frequently asked questions
Does it count business days?
No. This calculator adds calendar days (also called running or natural days): it includes Saturdays, Sundays and public holidays. If your deadline is measured in business or working days, this is not the right tool, because every country and process defines differently which days to skip.
How do I work out a deadline?
Set the base date to the day the clock starts, choose Add, and enter the length with its unit. For a one-year warranty from purchase, for instance: purchase date, add 1 year. Always double-check the original document, since some deadlines start counting the day after the event rather than on the event itself.
What happens when I add a month to January 31?
February has no 31st, so the date rolls back to the last available day: February 28 (or 29 in a leap year). In the same way, March 31 plus one month gives April 30. This is the most common convention, so you do not accidentally jump into a later month than intended.
Is the base date included in the count?
The operation starts from the day you enter as the base: the result is the date that falls after adding, or before subtracting, the interval. If your deadline says “starting the following day,” add one extra day to the amount.