How do I calculate my age from my date of birth?

Subtract your birth year from the current year. If your birthday has not happened yet this year, subtract one more year from that total. Then count the months and days separately using the same subtract-and-borrow method. This tool does all of that automatically. You just get the finished answer.

Can I calculate my age on a specific date?

Yes. Instead of leaving the calculate-as-of field on today, pick any date, past or future, and the tool works out your age as of that day. Use this for school enrollment cutoffs, immigration age requirements, or figuring out how old you will be on a future date like a wedding or retirement.

How does this handle leap years?

Leap years are built into the calendar math, not added as an afterthought. If someone's birthday is February 29, this tool treats it as February 28 in non-leap years for counting elapsed years, the same convention most legal and administrative systems use, while still calculating the exact day count correctly underneath.

What is chronological age?

Chronological age is the time that has passed since birth, measured in the calendar: years, months, and days. It is different from biological age, which estimates how old a body seems rather than how long a person has actually been alive. When a form or a doctor asks for your age, chronological age is what they mean.

How is Korean age different from international age?

Korean age starts at one, not zero, on the day someone is born, and everyone's age increases together on New Year's Day instead of on individual birthdays. Depending on birth month, that can put Korean age one or two years ahead of the international age most of the world uses. Toggle Also show Korean age to see both numbers at once.

How do I calculate age difference between two people?

Switch to the Age Difference tab, enter both birth dates, and the calculator returns the gap as years, months, and days, plus a flat day count if you need one number instead of three. It works the same way whether the gap is a few months or several decades.

How accurate is this online tool?

It is accurate to the day, using real calendar date math rather than a simplified days-divided-by-365 estimate. Leap years, varying month lengths, and the borrow-a-month logic are all handled correctly. The one thing it cannot account for is a wrong birth date typed in.

How do I calculate age in Excel?

Put the birth date in a cell, then wrap it in Excel's DATEDIF function with "Y", "YM", and "MD" as the unit arguments to pull out years, months, and days separately. The full formula is in the Excel section above. It is the same calendar-based approach this tool uses, just typed out manually.

Yes. Everything runs in your browser using JavaScript. Your birth date never leaves your device. There are no servers processing your information and no account required. You can disconnect from the internet after loading the page and the calculator still works.

Can I use this for my pet?

Yes. Click the Pet Age tab, choose dog or cat, and enter how many years old your pet is. The tool converts to human-equivalent age using the standard veterinary formula, not the old seven dog years equals one human year shortcut.

How Old Is Someone Born on December 21, 1985?

Current age

40 years, 8 months, 21 days

Total days: 14,874
Total weeks: 2,124
Completed months: 488
Total hours: 356,976
Total minutes: 21,418,560
Total seconds (live): 1,285,113,600

Next birthday: December 21, 2026 (101 days, turning 41)

Your date of birth

Date of birth

Calculate age as of

Also show Korean age

Results

Total days: 14,874
Total weeks: 2,124
Total months: 488
Total hours: 356,976
Seconds lived: 0

Elapsed time

Unit	As of September 11, 2026
Years, months, days	40 years, 8 months, 21 days
Total days	14,874
Total weeks (days ÷ 7)	2,124
Total hours	356,976
Total minutes	21,418,560
Total seconds (at page load)	1,285,113,600

Age milestones

Age	Date reached	Day of week
1	December 21, 1986	Sunday
5	December 21, 1990	Friday
10	December 21, 1995	Thursday
13	December 21, 1998	Monday
16	December 21, 2001	Friday
18	December 21, 2003	Sunday
21	December 21, 2006	Thursday
25	December 21, 2010	Tuesday
30	December 21, 2015	Monday
40	December 21, 2025	Sunday
50	December 21, 2035	Friday
60	December 21, 2045	Thursday
65	December 21, 2050	Wednesday
70	December 21, 2055	Tuesday
80	December 21, 2065	Monday
90	December 21, 2075	Saturday
100	December 21, 2085	Friday

Reference dates

Date	Exact age
Birth date (Saturday)	0 years, 0 months, 0 days
September 11, 2026	40 years, 8 months, 21 days
December 21, 2026	Turning 41
+1 mo → October 21, 2026	40 years, 10 months
+3 mo → December 21, 2026	41 years
+6 mo → March 21, 2027	41 years, 3 months
+9 mo → June 21, 2027	41 years, 6 months
+12 mo → September 21, 2027	41 years, 9 months

Week milestones from birth

Weeks after birth	Date
1	December 28, 1985
4	January 18, 1986
8	February 15, 1986
13	March 22, 1986
26	June 21, 1986
52	December 20, 1986

Related birth dates

50 related calculations
Birth date	Age today	Page
1985-12-20	40	View
1985-12-22	40	View
1985-12-19	40	View
1985-12-23	40	View
1985-12-18	40	View
1985-12-24	40	View
1985-12-14	40	View
1985-12-28	40	View
1985-12-07	40	View
1986-01-04	40	View
1985-11-21	40	View
1986-01-20	40	View
1985-10-22	40	View
1986-02-19	40	View
1985-09-22	40	View
1986-03-21	40	View
1975-12-21	50	View
1976-12-21	49	View
1977-12-21	48	View
1978-12-21	47	View
1979-12-21	46	View
1980-12-21	45	View
1981-12-21	44	View
1982-12-21	43	View
1983-12-21	42	View
1984-12-21	41	View
1986-12-21	39	View
1987-12-21	38	View
1988-12-21	37	View
1989-12-21	36	View
1990-12-21	35	View
1991-12-21	34	View
1992-12-21	33	View
1993-12-21	32	View
1994-12-21	31	View
1995-12-21	30	View
1985-12-01	40	View
1985-12-02	40	View
1985-12-03	40	View
1985-12-04	40	View
1985-12-05	40	View
1985-12-06	40	View
1985-12-08	40	View
1985-12-09	40	View
1985-12-10	40	View
1985-12-11	40	View
1985-12-12	40	View
1985-12-13	40	View
1985-12-15	40	View
1985-12-16	40	View

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How this is calculated

Exact age counts completed calendar years, months, and days between the birth date and the reference date.
Total days is the count of calendar days elapsed between those dates (including leap days).

Age = Reference date − Birth date

Weeks ≈ total days ÷ 7. Hours = total days × 24. Minutes = total hours × 60 (exact for whole-day spans).