date-time calculator

Age Calculator

Calculate exact calendar age, date differences, total elapsed days, next birthday, and the date a target age is reached.

Result

Choose dates and Calculate.

Formula version 1.0.0 · Gregorian calendar · UTC date-only arithmetic.

Calendar rules

How exact age is calculated

The calculator counts complete calendar years first, then complete months, then remaining days. It also reports the elapsed-day count.

Date-only arithmetic

Dates are parsed as Gregorian calendar dates in UTC. Time of day, daylight-saving changes, and local time zones are excluded.

Leap-day policy

A February 29 anniversary clamps to February 28 in a non-leap year. Legal birthday treatment may differ by jurisdiction.

Month ends

When a target month is shorter, the date clamps to that month's final valid day.

Privacy

Calculations run in the page. Avoid sharing personal birth dates unnecessarily.

How to use this calculator

Choose Age on a date, Difference between two ages/dates, or Date when an age is reached.
Enter each requested calendar date using its exact field label.
For Date when an age is reached, enter a nonnegative whole number in Target age in whole years.
Select Calculate, then review the calendar result, total days, assumptions, and calculation rules.

Symbols and units

Symbol	Meaning	Unit or domain
`birthDate`	Gregorian calendar birth date	YYYY-MM-DD date only
`asOfDate`	Date on which age is measured	YYYY-MM-DD date only
`years, months, days`	Complete constrained calendar units	Whole calendar units
`totalDays`	Elapsed UTC date-only days	Days
`targetYears`	Whole age used to find a target date	Nonnegative whole years

Checked examples

Age across a leap-day birthday

Inputs: Birth date: 2000-02-29; Age as of: 2023-02-28

Calculation: Count 23 complete years using the documented February 28 non-leap-year anniversary rule.

Result: 23 years, 0 months, 0 days

The calculator uses an explicit mathematical anniversary policy; legal treatment can differ.

Check: age-general.test.ts — handles leap-day anniversaries

Find the date a target age is reached

Inputs: Birth date: 1995-06-15; Target age: 30

Calculation: Add 30 constrained Gregorian calendar years to 1995-06-15.

Result: 2025-06-15

The target is a date-only result without time-zone or time-of-day assumptions.

Check: age-general.test.ts — finds the date at a target age

Assumptions, rounding, and limits

Dates use the proleptic Gregorian calendar and UTC date-only arithmetic; time of day and daylight-saving changes are excluded.
The as-of date cannot precede the birth date.
February 29 anniversaries clamp to February 28 in non-leap years; legal birthday rules may differ by jurisdiction.
Month-end operations clamp to the final valid day when a corresponding date does not exist.
Target age must be a nonnegative whole number within the formula's supported date range.
Birth dates are sensitive personal data; calculations run in the page and should not be shared unnecessarily.

Frequently asked questions

How is a February 29 birthday handled?

This calculator uses February 28 as the anniversary in a non-leap year. A legal or institutional rule may use another date.

Does the result depend on my time zone?

No. Inputs are date-only Gregorian values and calculations use UTC date arithmetic, not local clock time.

Why can total days differ from years times 365?

Calendar years vary because of leap days, and calendar months have different lengths.

Sources and review

TC39: Temporal.PlainDate — Date-only Gregorian arithmetic and constrained calendar operations
Internet Engineering Task Force: Date and Time on the Internet: Timestamps — Gregorian date syntax and leap-year definition

Formula version: age.general@1.0.0
Formula review: Equatara formula review
Editorial review: Equatara editorial review
Last verified: 2026-10-25
Applicability: Global educational calendar arithmetic; leap-day policy is disclosed and is not legal advice.
Next review: On material formula/source change, or by 2028-10-25.

Found a problem?

Report a formula, result, example, source, or accessibility issue through the calculator correction process. Do not include sensitive input values.

Calculate age and date differences