Anniversary & Days Elapsed Calculator
Calculate how many days it's been since a start date — like the day you started dating, a wedding anniversary, or a birthday — and find upcoming milestones like the 100th day, 1000th day, or Nth anniversary. You can also work out what date is N days or N years from a start date. Everything runs in your browser, so the dates you enter are never sent anywhere.
The start date is counted as "day 1".
The dates you enter are processed entirely in your browser and never sent anywhere.
Enter a start date and press "Calculate".
The start date and as-of date were swapped to calculate this.
- Today is
- Time elapsed
Milestones
- Result date
How to use
- Choose "Days elapsed & milestones" or "Calculate a future date".
- For elapsed/milestones, enter the start date. For a future date, enter the start date plus an amount and unit (days/years).
- Press "Calculate". For elapsed/milestones, the most recently reached milestone and upcoming ones are shown too.
How it works
How "day number" is counted
This tool counts the start date itself as "day 1". For example, if you started dating on January 1, then January 1 is day 1 and January 2 is day 2. This follows the common convention of counting the start date as day one (there's no single universal rule — some people count the day after the start date as day 1 instead).
"Days elapsed" is the as-of date (usually today) minus the start date, treating the start date as day 0. "Day number" is elapsed days + 1. Under this convention, the 100th-day milestone falls 99 days after the start date, and the 1000th-day milestone falls 999 days after.
Elapsed time breakdown (years / months / days)
The years/months/days breakdown uses the same calendar-based method used to calculate age from a birthdate — not a rough approximation based on dividing by 30 or 365.
Milestone list
Day-based milestones: 100, 200, 300, 500, 1000, 2000, 3000, 5000, 10000, 20000, 30000, and 50000 days. Year-based milestones: every year from 1 to 10, then 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, and 100 years. This list is based on patterns common to existing anniversary-counting tools.
Anniversary dates keep the same month and day as the start date, just in a later year. If that exact month/day doesn't exist in the target year (e.g. February 29 in a non-leap year), it's rounded down to the last day of that month.
Calculating a future date
Given a start date, this adds the specified number of days or years. When adding years, the month and day are kept the same, with the same end-of-month rounding described above.
Supported range
Dates from year 1 to year 9999 are supported. Dates outside this range, or results of a future-date calculation that fall outside this range, will show an error.
About data sent
All calculations happen entirely in your browser. No data, including the dates you enter, is ever sent to a server.
FAQ
How many days after the start date does the "100th day" fall?
99 days after the start date. This tool counts the start date as "day 1", so the 100th day is the 100th day in that count — 99 days after the start date. Likewise, the 1000th day falls 999 days after the start date.
Why is the start date counted as day 1?
Counting the start date (like the day a couple started dating, or a birth date) as "day 1" is the most common convention. It isn't a universal rule, though — some people count the day after the start date as day 1 instead. It's worth agreeing on the counting method with anyone else involved.
How is an anniversary date (1-year, 10-year, etc.) determined?
It's the same month and day as the start date, just N years later. For example, if the start date is January 1, 2026, the 1-year anniversary is January 1, 2027. If the start date is February 29 (a leap day) and the target year isn't a leap year, the date is rounded down to February 28.
Can I calculate this if the start date is in the future?
Yes. The order of the start date and as-of date doesn't cause an error — they're automatically swapped and the elapsed time is calculated from there, with a note shown that they were swapped.
Are the dates I enter sent anywhere?
No. All calculations happen entirely in your browser — nothing is sent to any external server or API.
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