Date Calculator Guide — D-day Countdown, Days Between Dates & Date ±N Days
Jun 23, 2026
How many days until the contract expires? When exactly is the 100-day anniversary? If you count back 280 days from the due date, what week is it now? Date math comes up more often than you'd expect — and counting squares on a calendar by hand almost always leads to an off-by-one error. The FreeSign Date Calculator lets you find the number of days between any two dates and look up a date N days forward or backward in seconds.
D-day countdown, days between two dates, date ±N days — free, no install or sign-up.
Calculate a date →When to use it — common situations
- D-day countdown — see exactly how many days remain until an exam, presentation, or trip departure.
- 100-day & anniversary milestones — find the exact date that falls 100 days or 1 year after a relationship start date or wedding date.
- Contract & savings maturity dates — add 12, 24, or 36 months to a start date to pinpoint when an agreement or deposit matures.
- Due date calculation — add 280 days (40 weeks) to the last menstrual period date to estimate a due date.
- Work deadline back-calculation — subtract 14 days or 10 business days from a deadline to set an internal target.
- Passport & visa expiry — add 10 years to an issue date to find the expiry, or check how many days remain on a visa.
Two modes — that's all you need
- Days between two dates — enter a start date and an end date; the calculator instantly shows the number of days between them. You can choose whether to include the start day (D-0 style) or not.
- Date ±N days from a base date — pick a reference date and enter a number of days to add or subtract. A positive number returns a future date; a negative number returns a past date.
Examples
| Situation | Input | Result |
|---|---|---|
| Couple's 100-day milestone | 2025-03-10 + 99 days | 2025-06-17 |
| Contract maturity | 2025-01-01 + 365 days | 2026-01-01 |
| Exam D-day | Today → 2026-08-20 | D-59 |
| Pregnancy due date | 2025-10-05 + 280 days | 2026-07-12 |
Tools that pair well with this one
When you also need to know someone's age — for a contract, form, or legal document — the Korean Age Calculator converts any date of birth into a current Korean age instantly. For a deeper look at how Korean age works and when to use it, see the Age Calculator Guide.
D-day · days between dates · date ±N days — free, no sign-up.
Calculate a date →Frequently asked questions
Q. Is the start date counted as Day 0 or Day 1?
Both conventions exist. Many people treat the target date itself as D-day (D-0), while others count from the day after the start. FreeSign's date calculator lets you choose either mode so you can match the convention you need.
Q. For a 100-day milestone, do I add 99 days or 100 days to the start date?
If you count the first day as Day 1, add 99 days to land on Day 100. If you count the first day as Day 0, add 100 days. Enable "include start date" in the calculator and it handles the offset automatically.
Q. Does it handle leap years and months with different lengths correctly?
Yes. The calculator is calendar-aware, so February 29 on leap years, 30-day months, and 31-day months are all accounted for automatically — no manual adjustment needed.
Q. Is it free? Do I need to create an account?
Completely free, no account required. Open it in your browser, calculate, and copy the result.