Everyday calculations
Age calculator
Calculate calendar age and elapsed days for a birth date and a chosen calculation date. Choose how leap-day birthdays are treated.
Works locally in your browser
How this tool works
Age is usually stated in completed calendar years, with months and days describing the remainder. This calculator keeps that calendar result separate from exact elapsed totals so the two measurements are not confused.
Inputs and definitions
- Birth date is the earlier Gregorian calendar date from which age is measured.
- Age as of is the date on which the age is required. It initially uses today's date from your device.
- Leap-day policy chooses either 28 February or 1 March as the anniversary of 29 February in a non-leap year.
Methodology
The method first counts completed anniversary years. It then advances by complete calendar months from that anniversary and counts the remaining days. Month-end dates are clamped to the last valid day when a corresponding day does not exist.
calendar age = completed years + completed months + remaining daysExact elapsed days are obtained from proleptic Gregorian day positions. Complete weeks are the elapsed-day total divided into groups of seven, with any leftover days shown separately. No clock time or timezone offset participates in either calculation.
Assumptions
- All dates use the proleptic Gregorian calendar, including dates before a country historically adopted that calendar.
- Today's date comes from the local calendar on the current device when the workspace opens.
- The next-birthday countdown is displayed only when the as-of date is today's date in this browser.
Worked examples
Use 28 February for a leap-day anniversary
Enter 29 February 2000 as the birth date, 28 February 2023 as the as-of date, and keep the 28 February non-leap-year policy.
Resolve the anniversary
Because 2023 has no 29 February, the chosen policy treats 28 February as that year's anniversary.
Count completed calendar years
The as-of date lands on the selected anniversary, so there is no remaining month or day component.
The calendar age is 23 years, 0 months and 0 days. Choosing 1 March instead would leave the twenty-third anniversary one day away.
Limitations and common mistakes
This is calendar arithmetic, not proof of identity or a legal determination of age. Laws, contracts, benefits, eligibility rules, and official systems can define when age changes differently, especially for a 29 February birth.
Frequently asked questions
Why is calendar age different from elapsed days divided by 365?
Calendar years have different lengths, and months range from 28 to 31 days. Dividing an elapsed total by a fixed number cannot reproduce anniversary-based years, months, and days in every case.
Does the calculator use my timezone?
Your device's local calendar supplies the initial today value. After that, entered dates are handled as date-only values, so timezone conversion cannot move them to the previous or next day.
Which leap-day birthday policy should I choose?
Use the convention relevant to your purpose. The default is 28 February, but the workspace also exposes 1 March because official and personal conventions are not universal.
Can this result establish legal age?
No. It provides transparent calendar arithmetic. Confirm the governing law, policy, or authority for legal eligibility and deadlines.
Sources
- Leap Years — National Institute of Standards and Technology (accessed )
- Temporal.PlainDate — Ecma International TC39 (accessed )
Method and content record
The method, examples and limitations describe how this tool works. Recorded checks do not establish independent professional review.
Record attribution, date and versions
Record attribution: ToolsOrigin calculation review; introduction and calendar guidance checked by Codex.
Record date: .
Content version 1.0.1. Engine version 1.0.0.