Explore birthday timing from a chosen reference date, including a selectable leap-day convention.
Calculate with the Birthday Calculator
Dates are validated as calendar components before calculation.
Birthday Calculator results
How to use the Birthday Calculator
- Enter the required calendar dates, times, or rule values.
- Choose the counting convention or mode where offered.
- Select Calculate or press Enter.
- Review the result, basis, and limitations before using it.
Worked example
A 29 February birthday can be observed on either 28 February or 1 March in a common year.
Assumptions
Date-only calculations use the proleptic Gregorian calendar and ignore clock time. Elapsed-time calculations and selected-zone conversions state their time basis separately.
Limitations
- Milestones are informational only.
- Local or legal birthday rules can use a different leap-day convention.
Frequently asked questions
What date basis does the Birthday Calculator use?
The result uses calendar dates unless the page explicitly describes elapsed time or a selected time zone.
Can daylight-saving time change calendar-day totals?
No. Date-only calculations use normalized UTC day numbers. Elapsed timestamp and time-zone calculations are explicitly handled separately.
Are entered dates stored?
No. Calculations run in your browser and the entered dates are not saved or sent anywhere.