Birthday Calculator
Find completed years of age and calendar days until the next birthday.
The interval at a glance
The timeline follows the entered dates. For counting conventions, holidays or rounding, see the explanation below.
About this calculation
Birthday questions involve two related counts: age in completed years and the wait until the next annual occurrence. This calculator reports both as numbers for a selected as-of date, including an explicit convention for February 29 births. Calculate birthday when scheduling dates and durations.
Formula
completed age = calendar years with the current year's birthday passed; days remaining = next birthday date − as-of date
Worked example
With Date of birth: 1995-06-15, As-of date: 2026-10-22, completed age is 31 years.
Inputs and units
Enter a date of birth and an as-of date on or after it. The as-of date is a deliberate input, so a shared link remains meaningful later rather than silently changing when someone opens it on another day.
- Date of birth: Enter a valid Gregorian calendar date for the person's birth.
- As-of date: Choose the date on which to count age and the next birthday.
Calculation and worked example
Subtract the birth year from the as-of year, then remove one if that year's birthday has not occurred. Construct the next occurrence of the same month and day and count midnight-to-midnight UTC calendar days from the as-of date In compact notation, completed age = calendar years with the current year's birthday passed; days remaining = next birthday date − as-of date.
Someone born June 15, 1995 is 31 completed years old on October 22, 2026. Their next birthday is June 15, 2027, so the second output counts the calendar days from October 22 to that date.
How to use the answer
A zero-day wait means the as-of date itself is the birthday. Completed age changes only on the annual birthday; it does not round up because the next birthday is close.
Use the as-of date to answer an historical question, such as age at a past event, or to plan an upcoming birthday. For legal eligibility rules, check the applicable jurisdiction's date and age conventions separately.
Assumptions and checks
Dates use the proleptic Gregorian calendar and UTC midnights to avoid local daylight-saving anomalies. A February 29 birthday is treated as March 1 in years without February 29, because that date rolls forward on construction.
Do not calculate age by dividing days lived by 365; leap years and whether the birthday has occurred matter. A birth date later than the as-of date is invalid for this age calculation.
Move the as-of date one day before and then to the birthday: completed age should increase by one and days-until should reach zero. Test February 29 separately if that convention matters.
Common questions
What does the completed age result represent?
Completed age is the number of full birthday anniversaries reached on the chosen as-of date, using March 1 for leap-day births in non-leap years.
What should I check before relying on this result?
Verify both dates and the February 29 convention. The tool gives arithmetic counts, not a legal determination of age.
Inputs
- Date of birth
- As-of date