Adding days to a date sounds trivial until the span crosses a month or year boundary, or a February in a leap year. This calculator counts real calendar days, so the answer is correct whatever it crosses. Adding 100 days to 9 August 2026 lands on Tuesday 17 November 2026.
The result also tells you the weekday, which is usually the point. Deadlines that fall on a Saturday get treated differently from ones that fall on a Wednesday, and a 90-day payment term can quietly become 92 if nobody checks.
Use a negative number to count backwards. The maths is symmetrical, so subtracting 30 days from a date and then adding 30 back always returns the original date — something month-based arithmetic cannot promise.
The formula
S- Start date
N- Days to add (negative counts back)
R- Resulting calendar date
How to use this calculator
- Enter the start date as YYYY-MM-DD.
- Enter how many days to add — use a minus sign to go backwards.
- Choose whether the start date itself counts as day one.
- Read the result date, its weekday, and the equivalent in weeks.
Worked examples
100 days after 9 August 2026
The result is Tuesday 17 November 2026. That is 14.29 weeks, or 14 whole weeks plus 2 days, which is why the weekday moves from Sunday to Tuesday.
Crossing a leap day
Adding 30 days to 15 February 2024 gives 16 March 2024, because 2024 had a 29 February. The same sum in 2026 gives 17 March 2026 — one calendar day later in March.
How to read your score
Frequently asked questions
What does inclusive counting mean?
Inclusive counting treats the start date as day one, so "3 days from Monday" ends on Wednesday rather than Thursday. Contracts and court deadlines vary on this, so the option is explicit here.
Can I add a negative number of days?
Yes. A negative value counts backwards through the calendar and the result is exact, including across year boundaries and leap days.
Why does the weekday shift the way it does?
Because weekdays repeat every seven days, the shift equals the remainder when your day count is divided by seven. 100 divided by 7 leaves 2, so the weekday moves forward 2 places.
Is the calendar breakdown the same as the day count?
No, and that is the point. The day count is absolute; the breakdown into years, months and days depends on which months the span happens to cross.
Day offsets from 9 August 2026
| Days added | Result date | Weekday | Weeks |
|---|---|---|---|
| 7 | 16 August 2026 | Sunday | 1.00 |
| 14 | 23 August 2026 | Sunday | 2.00 |
| 30 | 8 September 2026 | Tuesday | 4.29 |
| 45 | 23 September 2026 | Wednesday | 6.43 |
| 60 | 8 October 2026 | Thursday | 8.57 |
| 90 | 7 November 2026 | Saturday | 12.86 |
| 100 | 17 November 2026 | Tuesday | 14.29 |
| 180 | 5 February 2027 | Friday | 25.71 |
| 365 | 9 August 2027 | Monday | 52.14 |
Every figure is a real calendar count from 9 August 2026, leap days included.