How is age calculated?

Age is the time from your date of birth to a comparison date (today by default), counted in whole years, then whole months, then the leftover days. A month is counted as reaching the same day of the next month — so from the 15th to the 15th is one month — and a year is twelve such months.

How do you count age across months of different lengths?

Whole months are counted by the calendar, not by a fixed number of days. Going from January 31 forward one month lands on the last day of February, and any days past that are counted on top. This matches the common, everyday way people count age.

What happens to a February 29 birthday in a non-leap year?

Because February 28 is the last day of the month in a common year, a person born on February 29 completes another full year on February 28. This calculator treats February 28 as the birthday in non-leap years.

Can I find the difference between two arbitrary dates?

Yes. Set the date of birth to the earlier date and "Age at the Date of" to the later date, and the result is the interval between them — in years, months, and days, plus total weeks, days, hours, minutes, and seconds.

Does this calculator account for the time of day?

No. It works in whole calendar days, so the hours, minutes, and seconds are exact multiples of full days rather than counted from a specific birth time.

Age & Birthday Calculator — Chronological Age and Dates

Find your exact age, or the interval between any two dates, in years, months, and days. Enter your date of birth and, optionally, the date to measure to — leave it blank to use today. The same span is also shown in total months, weeks, days, hours, minutes, and seconds.

Example

Field	Value
Date of Birth	January 1, 1990
Age at the Date of	June 17, 2026
Age	36 years, 5 months, 16 days
Or in months	437 months and 16 days
Or in weeks	1,902 weeks and 2 days
Or in days	13,316 days

Examples

Someone born January 1, 1990 on June 17, 2026

From January 1, 1990 to June 17, 2026 is 36 years, 5 months, and 16 days — the same as 437 months and 16 days, or 13,316 days, or 1,150,502,400 seconds.

A February 29 birthday

A person born February 29, 2000 reaches 25 years, 0 months, 0 days on February 28, 2025, because February 28 is the last day of the month in that common year.

Formula

Age uses the common age system: count whole years and months by the calendar, then add the remaining days.

1. months = (asOfYear − birthYear) × 12 + (asOfMonth − birthMonth)
   if birth + months lands after the comparison date, subtract one month
2. years  = months ÷ 12   (whole)      remaining months = months mod 12
3. days   = comparison date − (birth + whole months)

Whole months are added with day clamping, so January 31 plus one month is the last day of February, and a February 29 birthday completes a year on February 28 in a common year.

To measure the gap between two events, put the earlier date in "Date of Birth" and the later one in "Age at the Date of".

Calendar assumptions and limits

The result counts complete calendar years, months, and days between two civil dates. Month lengths and leap years are handled as calendar values; they are not replaced with a fixed 30-day month or 365-day year. A birth time, time zone, or legal age rule can change the answer for official purposes, so use the date and rule required by the relevant authority.

For a related calculation, use our age difference calculator.

What is chronological age?

Chronological age is the calendar interval from your birth date to the comparison date. Use the Age tab and enter a specific comparison date for an application, anniversary, or past event; leave it blank for today. It is different from an estimate of biological development or health. The calculator does not infer either from a birth date.

Whole years and months are counted first, then remaining days. Months have different lengths, so dividing elapsed days by 365 does not reproduce a calendar age. For a January 31, 2024 birth and March 1, 2024 comparison, the result is one month and one day: the monthly anniversary is clamped to February 29. February 29 births complete a calendar year on February 28 in a non-leap year under this age convention.

Birthday countdown and milestone dates

Choose Birthday to find the weekday of birth, days until the next birthday, the age on that date, the next ten birthday weekdays, and upcoming milestone birthdays. A countdown of zero means the birthday is today; it does not skip ahead to next year. These are date-only counts, so they do not depend on the time of birth or the current hour.

For a February 29 birthday, choose February 28 or March 1 as the non-leap-year celebration date. Leap years continue to use February 29. This option controls birthday planning, not the chronological-age convention or any jurisdiction's legal-age rule. Milestones are shown only when they are on or after the comparison date.

Birthstone traditions

The birthday result includes modern birthstone options by birth month using the Gemological Institute of America's birthstone guide. For example, January is garnet, April is diamond, and June includes pearl, alexandrite and moonstone. These are cultural associations, not predictions about personality or health.

This tool focuses on dates, milestones and birthstone traditions. It does not provide music charts, movie rankings, celebrity birthday databases or astrological readings. Use the Date Calculator for custom weekdays or time-of-day intervals.

Which generation does your birth year belong to?

Use the Generation Calculator to look up a birth-year cohort and compare US and McCrindle year ranges. Generation labels depend on the chosen convention; your exact chronological age is a separate calculation.

Age Calculator

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