How to Calculate Age from a Date of Birth

Age calculation seems simple but has several edge cases — leap year birthdays, timezone differences, and whether you count completed years vs calendar years. This guide covers the correct approach.

Basic Method: Completed Years

The most common definition of age is completed years of life:

Start with the current date.
Subtract the birth year from the current year.
If the birthday hasn't occurred yet this year, subtract 1.

Example: Born 18 October 1990, today is 18 April 2026:

2026 − 1990 = 36
Birthday (Oct 18) has not occurred yet in 2026 (it's April)
Age = 36 − 1 = 35 years old

Calculating Age in Months and Days

For more precision:

Total months = (Current year × 12 + Current month)
             − (Birth year × 12 + Birth month)

Then subtract 1 if the day of month hasn't been reached yet this month.

Example: Born 15 June 1990, today is 18 April 2026:

Months = (2026×12 + 4) − (1990×12 + 6)
= 24316 − 23886 = 430 months
April 18 ≥ June 15? No → subtract 1 → 429 months
= 35 years, 9 months, 3 days

Leap Year Birthdays (29 February)

People born on 29 February can only celebrate on their exact birth date in leap years. In non-leap years, most countries legally define the birthday as:

28 February (UK, Hong Kong)
1 March (some other jurisdictions)

For age calculation, the convention is: if 29 Feb does not exist this year, the birthday is considered to occur on 28 Feb.

Age in Days

Age in days = (Today's date) − (Date of birth)

Accounting for leap years, the average year is 365.2425 days. A 30-year-old has lived approximately:

30 × 365.2425 ≈ 10,957 days

To find exact days, count the calendar days between the two dates directly — don't multiply and round.

Timezone Considerations

If you were born in one timezone and now live in another, "today" might differ. Someone born just after midnight in Tokyo is born the "previous day" in New York. For legal and medical purposes, age is typically calculated using local time at the place of birth.

Age in Different Cultures

East Asian reckoning (traditional): In China, Japan, and Korea, a person is considered 1 year old at birth and gains a year every New Year. So a baby born in December could be "2 years old" in January, despite being only a month old in Western counting.
Korean system: Still used informally; the government switched to the international standard in 2023.

Using a Date Difference for Age

Any date difference tool that returns years, months, and days between two dates — anchored to today as the end date — gives you your age. Use the Age Calculator to see your exact age in years, months, days, hours, and total days lived.