When to use this calculator
Find completed years of age between a birth date and an as-of date. For 29 February births, completed years advance on 1 March in non-leap years. This is a calendar convention, not a legal-age ruling.
Explore the calendar dates & age calculators for other questions in this topic. For a related calculation, use the Age in Days Calculator: Count complete calendar days between a birth date and an as-of date.
Use the date the age is needed for
An application, anniversary or historical record may require age on a particular day rather than today. Enter that day as the as-of date. A birthday tomorrow has not yet been completed today. For a 29 February birth, this tool advances the completed year on 1 March in non-leap years.
PUTTING IT INTO PRACTICE
Age: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Birth date | 1990-06-15 |
| As-of date | 2026-10-17 |
| Result | 36 years |
- Check the input labels and enter the example values above.
- Apply the relationship: Subtract birth year, then subtract one if the birthday has not yet occurred..
- The result is 36 years, rounded for display.
How to interpret your result
For 29 February births, completed years advance on 1 March in non-leap years. This is a calendar convention, not a legal-age ruling.
Choose whether an interval includes the start or end boundary. Month and year additions use explicitly stated clamping rules when a destination day does not exist. Date-only arithmetic is kept independent of local daylight-saving hour shifts.
Frequently asked questions
Does this establish legal age for an application?
No. Legal and administrative rules can use different conventions, especially for leap-day births. Confirm the rule with the organisation requesting the age.
Does adding one month always add the same number of days?
No. Calendar months vary in length. A day-count addition and a calendar-month addition solve different questions.
When is the age calculator useful?
Find completed years of age between a birth date and an as-of date. For 29 February births, completed years advance on 1 March in non-leap years. This is a calendar convention, not a legal-age ruling.
How do I use the age calculator?
Enter birth date, as-of date. The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Subtract birth year, then subtract one if the birthday has not yet occurred..
What should I check before using the result?
For 29 February births, completed years advance on 1 March in non-leap years. This is a calendar convention, not a legal-age ruling.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
Further reading
For background on this topic, see MDN: date values and calendar handling ↗. This is a topic reference, not an external review or endorsement of this tool.
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