Anniversary calculator
Count the days since a start date, see the countdown to the next anniversary, and get the exact dates of the 100, 200, 520, 1000 and 3650 day milestones.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Dates
Everything is counted in calendar days, independent of time zone and daylight saving; a 29 February anniversary falls on 28 February in common years.
Result
Pick a start date to see the days passed, the next anniversary and the milestones.
What this tool does
- Track a relationship, wedding or work anniversary: enter the start date to see the days passed, the date of the next anniversary and how long is left.
- Plan a gift for a 100-day, 520-day or 1000-day milestone by looking up the exact calendar date first.
- Replace counting on your fingers: “how many days since 1 January 2020?” is answered with leap years and month lengths already handled.
- Check a cross-year anniversary (say 1 January) by reading the countdown to the next one and planning around it.
Example
Input
Start date 2020-01-01, count up to 2024-01-02
Output
Days passed 1462, weeks passed 208, 4 completed years; next anniversary 2025-01-01 (5 years, in 365 days); day 100 is 2020-04-10, day 200 is 2020-07-19, day 520 is 2021-06-04, day 1000 is 2022-09-27 and day 3650 is 2029-12-29
2020 is a leap year, so the first four years add up to 1461 days; 2024-01-02 is one more.
Frequently asked questions
Why does a 29 February anniversary show 28 February in some years?
Common years have no 29 February, so the anniversary is clamped to 28 February, matching what most calendar apps do. In leap years such as 2024 and 2028 it moves back to 29 February, which is why the countdown differs slightly in those years.
How are the days counted, and does my time zone matter?
It does not. Every calculation subtracts calendar days (treating each date as UTC midnight), so only the year, month and day are used. “2020-01-01 to 2024-01-01” is 1461 days wherever you are.
What if the start date and the target date are the same day?
Days passed and completed years are both 0, and the next anniversary is the same day next year (28 February when the start date is 29 February). If the target date is earlier than the start date the tool reports an error instead of a negative number.
Does “days passed” include the start day?
No. It is the number of whole days between the two dates, which is the same as the N in “day N”. If you want to count both the first and last day, add 1.
What happens if I leave the target date empty?
The page fills in the current date when it loads, and you can change it to any day to preview a future countdown, for example to see when next year’s anniversary falls.
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