What this calculator tells you
Give it a date of birth and it returns your age in completed years, your age in years and months, the number of days you have lived, and how long until your next birthday. You can also pick a different "as of" date, which turns it into a planning tool: how old will a child be on the first day of school, how old will you be at a retirement date or a wedding, how old was a relative on a date written in an old document. A second panel counts the days to or since any target date. Everything is calculated in your browser from the three numbers in a calendar date, so nothing is uploaded.
How age is counted
In most of the world you are zero years old at birth and gain exactly one year on each birthday. The rule is easy to say and easy to get wrong in code: age = this year - birth year, minus one if this year's birthday has not happened yet. On the birthday itself you are already the new age.
The calculator also shows two related numbers so you can tell them apart. The year difference is simply this year minus the birth year, with no regard for the birthday; it is what some forms mean by "age at the end of the year" and what gets used for cohort or age-group tables. The East Asian reckoned age follows the traditional system used historically in Korea, Japan (kazoedoshi) and China (xusui): a baby is one year old at birth and everyone gains a year together at the new year. In that system the number is the year difference plus one. The traditional count survives mainly in customs and in the way some older relatives talk about age. Korea moved its legal default to the international way of counting in June 2023, and the other two countries also use the standard count for official purposes.
The same people, three numbers
The table below is computed when the page is built, with the same formula as the calculator, for a calendar date of 2026-10-13. Six birthdays were chosen to sit at the awkward edges.
| Date of birth | Age (standard) | Year difference | Reckoned age |
|---|---|---|---|
| 2000-01-01 | 26 | 26 | 27 |
| 2000-05-15 | 26 | 26 | 27 |
| 2000-10-13 | 26 | 26 | 27 |
| 2000-10-14 | 25 | 26 | 27 |
| 2000-12-31 | 25 | 26 | 27 |
| 2026-01-05 | 0 | 0 | 1 |
Two rows deserve attention. Someone born on 2000-10-13 is 26 on exactly the reference day, but a person born one day later, on 2000-10-14, is still 25. And a person born on New Year's Eve (2000-12-31) is 25 by the standard count yet has a year difference of 26, which is why age-group statistics and "how old are you" in conversation can disagree for almost a full year. A baby born on 2026-01-05 is 0 in the standard count and 1 in the reckoned count.
Worked examples
Example 1. Born 2000-05-15, as of 2026-10-13. 2026 minus 2000 is 26, and May 15 has already passed this year, so the age is 26 and the reckoned age is 27.
Example 2. Born 2001-12-31, as of 2026-01-01. The subtraction gives 25, but the 31 December birthday has not arrived yet, so the age is 24 and the year difference is 25. One calendar day earlier or later in the year can change the answer by a year, which is the usual reason people get this wrong by hand.
Example 3. Born 1995-03-20, as of 2026-03-19. 2026 - 1995 = 31, and the birthday is tomorrow, so the age is 30 and the countdown says 1 day to go. On 2026-03-20 the same calculation returns 31 and "today is your birthday".
Born on February 29
A person born on a leap day has no calendar birthday in three years out of four. Which day counts as the birthday in a common year is a convention, not a mathematical fact, and it differs between legal systems, companies and software. The two usual choices are February 28 (the birthday arrives at the end of February) and March 1 (the first day after the missing date). The calculator lets you pick; February 28 is the default. The table shows a person born on 2008-02-29 under both conventions.
| As of | Feb 28 convention | Mar 1 convention |
|---|---|---|
| 2026-02-27 | 17 | 17 |
| 2026-02-28 | 18 | 17 |
| 2026-03-01 | 18 | 18 |
| 2028-02-28 | 19 | 19 |
| 2028-02-29 | 20 | 20 |
In 2026, which is not a leap year, the two conventions disagree on exactly one day, February 28. In 2028 February 29 exists, so both agree and the person turns 20 on that day. If a leap-day birth matters for a licence, a contract or an age limit, check what the issuing body says; do not assume.
Day-count milestones
Counting days instead of years gives some round numbers that fall in odd places. The table lists the date on which a person born on 2000-01-01 has lived 5,000, 10,000, 15,000, 20,000 and 25,000 days (birth date plus N days, so day 0 is the birthday). Years are approximate because 365.2425 days is the Gregorian average.
| Days lived | Date reached | Years (approx.) |
|---|---|---|
| 5,000 | 2013-09-09 | 13.7 |
| 10,000 | 2027-05-19 | 27.4 |
| 15,000 | 2041-01-25 | 41.1 |
| 20,000 | 2054-10-04 | 54.8 |
| 25,000 | 2068-06-12 | 68.4 |
The 10,000th day arrives at roughly 27 years and 4 months, so it is a neat excuse for a small celebration in the middle of a year. The calculator's "Days lived" chip uses the same calendar-day subtraction, which ignores clock times and time zones.
Common mistakes and limits
- Subtracting years only. That gives the year difference, not the age. It is wrong for everyone whose birthday has not come yet this year.
- Treating the birthday as "not yet". The new age starts at midnight at the beginning of the birthday. Some legal systems put the change slightly earlier, so rely on the issuing authority for official thresholds.
- Time-zone drift. Parsing a date string as UTC and then reading it in local time can shift the day by one. This page computes with year, month and day numbers only, so the result does not move with your time zone.
- Mixing months. "Years and months" counts a full month only when the day of the month has been reached; a person born on the 31st completes a month at the end of a shorter month.
- Lunar and other calendars. The input is a Gregorian date. Convert a lunar or other calendar birthday first.
- Time of birth. Hours and minutes are not used, so twins born either side of midnight have different dates but the calculator only looks at the date.
When not to rely on it
This is a convenience calculator. For insurance, pensions, school admission, voting, age verification, visas or any legal threshold, the rule written by the organisation or authority involved decides, including its cut-off date and its treatment of leap-day births. Insurers in particular may use a policy-specific "age nearest birthday" or "age last birthday" method. Nothing here is legal advice.
Using the result
- On forms, write the standard age unless the form defines age differently.
- To compare two people, use the Age Difference Calculator, which gives years, months, days and the total number of days between two birth dates.
- For longer countdowns with more options, open the D-Day Countdown Calculator.
- For the animal side of the family, the Pet Age Calculator shows how a pet's years compare with human ones.
- For a lighter use of the same date, the Chinese Zodiac Calculator looks up an animal sign from the birth year.
About the D-Day panel
The D-Day panel compares calendar dates with today's date. A future date shows as D-n, a past date as D+n and today as D-Day. Because only the date is compared, the result is the same whether you check at 1 a.m. or 11 p.m. If you need to count the start date as day 1, exclude weekends or work with ranges, use the longer D-Day tool linked above.
Frequently Asked Questions
How is age calculated?
Subtract the birth year from the year of the reference date, then subtract one more if the birthday has not yet occurred that year. On the birthday itself the new age already applies.
What is the "East Asian reckoned age" chip?
It is the traditional count in which a baby is one at birth and everyone adds a year at the new year. The number shown equals the year difference plus one. It is a cultural reference, not the age used on official forms in most places.
When does a leap-day baby get older in non-leap years?
There is no universal rule. Some systems use February 28 and others March 1, so the calculator offers both. The results differ on one day each common year.
Can I use a future date as the "as of" date?
Yes. Enter the future date to see how old someone will be then. The date of birth must not be later than the reference date or the calculator will ask you to check it.
How does the D-Day counter show past dates?
A date that has already passed appears as D+n and an upcoming date as D-n, with the number of days counted by calendar date.