Birthday Calculator
Find the next birthday and the number of days remaining from a reference date you choose.
Result for the values shown.
Calculations run in your browser.
What to enter
| Input | Meaning and units |
|---|---|
| Birth date | Gregorian calendar date in YYYY-MM-DD format; years 0001 through 9999. |
| As-of date | Gregorian calendar date in YYYY-MM-DD format; years 0001 through 9999. |
Understanding your result
Calendar results use the stated counting and anniversary convention. Complete months and years differ from fixed day counts.
Common mistakes
- Counting both end dates when the method excludes a boundary, or assuming all months have equal length.
Check your calculation
- Test a short known interval and a leap-day boundary; for working days, verify your holiday list.
Calculation checks, sources and review limits
How many days are left until the next birthday?
Find the next birthday and the number of days remaining from a reference date you choose.
Common uses
- Find the next birthday and the number of days remaining from a reference date you choose.
How it works
Construct this-year birthday, clamping Feb29 to Feb28; if before asOf date advance one year; days=ordinal difference. Calendar arithmetic uses the proleptic Gregorian calendar. Explicit calendar dates are independent of local daylight-saving changes. Month/year additions clamp to the last valid day of the destination month. Timestamp arithmetic requires a stated UTC offset and excludes leap seconds.
Worked example
Enter Birth date: 2000-06-15; As-of date: 2024-06-14. The result is Next birthday: 2024-06-15; Days until: 1.
FAQ
How many days are left until the next birthday?
Construct this-year birthday, clamping Feb29 to Feb28; if before asOf date advance one year; days=ordinal difference. Calendar arithmetic uses the proleptic Gregorian calendar. Explicit calendar dates are independent of local daylight-saving changes. Month/year additions clamp to the last valid day of the destination month. Timestamp arithmetic requires a stated UTC offset and excludes leap seconds.
What assumptions and limits apply?
On birthday returns zero days. Feb29 convention explicitly shown.