Assumptions
- Countdowns use today's date on this device.
- Day milestones count the start date as day 1 by default; uncheck to use start + n.
- Yearly anniversaries keep the same month and day; Feb 29 becomes Feb 28 in non-leap years.
How it is calculated
- Countdown = target − today. Positive → D-n, zero → D-Day, negative → D+n.
- Day milestone = start + (n − 1) when the start date is day 1, otherwise start + n.
- Anniversary = same month and day, year + n; clamped to Feb 28 when Feb 29 does not exist.
- All dates are normalized to noon UTC, so daylight-saving changes never shift a count by one.
Examples
| Situation | Input | Result |
|---|---|---|
| 100 days together | Started 2026-01-01, start = day 1 | 2026-04-10 (Fri) |
| Baby\'s 100th day | Born 2026-06-01 | 2026-09-08 (Tue) |
| 1000 days sober | Started 2024-01-01 | 2026-09-26 (Sat) |
| Days until the bar exam | Target 2027-02-23, today 2026-09-06 | D-170 |
| Store\'s 1st anniversary | Opened 2025-09-06 | 2026-09-06 (Sun) |
Common mistakes
- Apps disagree by one day on milestones because of the day-1 convention. Agree on one before planning a party.
- The countdown follows your device clock. Traveling across the date line changes "today".
- Check the weekday column before booking a venue — anniversaries drift one weekday per year (two after a leap day).
FAQ
Is the 100th day counted from the start date or the day after?
Both conventions exist. With "count the start date as day 1" checked, January 1 is day 1 and the 100th day is April 10 (start + 99). Unchecked, it is start + 100 = April 11. Korean and some East Asian traditions count the first day as day 1; most English-speaking countdown apps do not.
What do D-1 and D+1 mean?
D-1 is the day before the target, D-Day is the day itself, D+1 is the day after. This tool shows D-n for future dates and D+n for past dates, relative to today.
Why is the anniversary the same calendar date while the 100th day shifts by one?
Anniversaries are date-based (same month and day each year; Feb 29 falls on Feb 28 in non-leap years). Day milestones are count-based, so the "day 1" convention matters.
How long is 1000 days?
About 2 years, 8 months and 26 days — one day more or less depending on how many leap days fall inside. The exact date is in the table.
Is anything saved?
No. Everything runs in your browser and nothing is sent to a server. Refreshing clears the inputs; bookmark the page if you want to come back.