How to use
- Pick a start date. Today’s date is filled in when the page opens.
- Choose add or subtract, then enter an amount and a unit (days, weeks, months or years). The resulting date and weekday appear instantly.
- Use the quick buttons — +100 days, +1,000 days, +1 year and more — to fill everything in with one tap.
How each unit works
| Unit | Method | Example (from Oct 5, 2026) |
|---|---|---|
| Days | Calendar days are added one by one | +100 days → Wed, Jan 13, 2027 |
| Weeks | 7 × N days | +2 weeks → Mon, Oct 19, 2026 |
| Months | Same day of the month, N months later | +3 months → Tue, Jan 5, 2027 |
| Years | Same month and day, N years later | +1 year → Tue, Oct 5, 2027 |
Going back 100 days from October 5, 2026 lands on June 27, 2026; 1,000 days ahead is July 1, 2029.
Month-end rule
When you add months or years and the resulting month does not have that day, the date moves to the last day of that month.
| Calculation | Result |
|---|---|
| Jan 31, 2026 + 1 month | Feb 28, 2026 (common year) |
| Jan 31, 2024 + 1 month | Feb 29, 2024 (leap year) |
| May 31, 2026 − 1 month | Apr 30, 2026 |
| Feb 29, 2024 + 1 year | Feb 28, 2025 |
A note appears under the result whenever this adjustment happens. Banks, insurers and billing systems sometimes use different month-end rules, so double-check due dates in your contract.
FAQ
Is a month treated as 30 days?
No. Months follow the calendar, so one month can be 28 to 31 days long. If you need a fixed number of days, switch the unit to days.
What is the difference between “100 days later” and “day 100”?
“100 days later” adds 100 days to the start date. “Day 100” counts the start date as day 1, so it is 99 days later. When you add days, the tool shows both. For a full milestone list, try the anniversary mode of the D-Day calculator.
Can I see how many days apart two dates are?
The “Days between” figure shows it for your result. To compare any two dates directly, use the date difference calculator.