What is the difference between age since birth and gestational age?

Age since birth measures time from the delivery date. Gestational age measures time from the last menstrual period before birth, about 40 weeks. Newborns have an age since birth of zero but a gestational age of 38 to 42 weeks.

How do I calculate age for someone born on 29 February?

On 28 February in a non-leap year, they are one day short of their birthday. On 1 March, they have turned that age. Age advances each year, not every four years. Use 1 March as the birthday in common years.

Why does my age in months not match a simple calendar count?

Because months have different lengths, from 28 to 31 days. A simple subtraction of month numbers fails when the current day is earlier in the month than the birth day. Borrowing from the previous month corrects this.

Can I use a duration calculator for age?

Yes. Enter the birth date as the start date and today’s date as the end date. A well-built duration calculator accounts for month lengths and leap years automatically.

What is the most common mistake when calculating age?

Forgetting to borrow days when the current day is smaller than the birth day. This produces a negative day count or an incorrect month count. Always check whether borrowing is needed.

Who should use gestational age instead of age since birth?

Medical professionals assessing newborn development, and anyone calculating pregnancy due dates. Gestational age is the standard in prenatal care. Everyone else calculating their own or someone else’s age should use age since birth.

Age Calculator: Years, Months, Days Precisely

Your exact age in years, months, and days is rarely a clean round number. Here is how to calculate it precisely, why the months matter, and what to do if you were born on 29 February.

How to Calculate Your Exact Age in Years, Months, and Days

If you were born on 15 March 2000, on 15 March 2026 you are exactly 26 years, 0 months, and 0 days old. But on 14 March 2026, you are 25 years, 11 months, and 27 days old. The difference between those two dates is not one day of age. It is a gap of four days in the month count. That happens because months have different lengths, and automatic subtraction without accounting for the birth date’s day of month gives the wrong answer.

To get your exact age, subtract the birth date from today’s date one component at a time. When subtraction fails, borrow from the next unit up.

Step 1: Subtract Days First

Take today’s day-of-month number and subtract your birth day-of-month number. If today’s day is larger, you are done with days. If it is smaller, borrow one month from the month count, then add the number of days in the previous month to today’s day before subtracting.

Example: Born 20 January 2000. Today is 14 March 2026. - Days: 14 minus 20 fails. Borrow one month from March, making the month count 2 instead of 3. The previous month is February. 2026 is not a leap year, so February has 28 days. The calculation becomes (14 + 28) minus 20. Result: 22 days.

Why this goes wrong: Many people subtract the birth day from the current day without borrowing. That gives 14 minus 20, a meaningless negative number. The correct result is 22 days.

Step 2: Subtract Months Next

After borrowing for days, take the adjusted month number and subtract the birth month number. If the adjusted month is larger, you are done. If it is smaller, borrow one year from the year count, then add 12 months before subtracting.

Example continued: After borrowing, the adjusted month is 2. Birth month is 1 (January). 2 minus 1 equals 1 month.

If the birth month were June (6): 2 minus 6 fails. Borrow one year from 2026, making the year 2025. Add 12 months: (2 + 12) minus 6 equals 8 months.

Step 3: Subtract Years Last

After borrowing for months, subtract the birth year from the adjusted year.

Example continued: Adjusted year is 2026. No borrow was needed for months. 2026 minus 2000 equals 26 years.

Full result for 20 January 2000 to 14 March 2026: 26 years, 1 month, 22 days.

Handling Months with Different Lengths

The borrowing step for days depends on the length of the month that precedes the current month. Months vary from 28 to 31 days. A duration tool must know the exact month and year it is borrowing from.

Borrowing from January: the previous month is December, 31 days.
Borrowing from March: the previous month is February, 28 or 29 days depending on the year.
Borrowing from August: the previous month is July, 31 days.

Common error: Assuming every month has 30 days. That shortcut works for rough estimates but fails for precise age. A person born on 31 January calculating age on 28 February in a non-leap year would get a negative day count. The correct method: borrow one month from February (which becomes January), giving 28 plus 31 minus 31. Result: 28 days.

Born on 29 February? Here Is How Your Age Works

For a 29 February birth, the standard rule: on 28 February in a non-leap year, you are exactly one day short of your birthday. On 1 March, you are one day past it. Most legal systems treat 1 March as the equivalent birthday in non-leap years. For precise calculation, 28 February is the day before your birthday and 1 March is the day after.

To calculate age for a 29 February birth: - If today is 28 February in a common year: you are exactly one day younger than your next birthday. The day count since your last birthday will show 365 days if the previous year was a common year. - If today is 1 March in a common year: you have just turned that age. The month and year increments happen on 1 March, not 28 February.

Wrong advice you should ignore: Some sources claim people born on 29 February age only every four years. That is false. Age advances each year, just on 1 March instead of 29 February. A person born 29 February 2000 is 26 years old on 1 March 2026.

Use the Duration Calculator to Get Your Age Instantly

Open the duration calculator. Enter your birth date as the start date. Enter today’s date as the end date. The tool runs the three-step borrowing process automatically and handles month lengths and leap years without manual arithmetic.

For age, you want time since birth. That is the standard output of any well-built date duration tool.

Gestational Age vs Age Since Birth

Two different age measurements exist for the same person. Age since birth counts from the delivery date. Gestational age counts from the first day of the last menstrual period before birth. Gestational age is about 40 weeks at term, roughly 9 calendar months and 6 to 7 days.

Why this matters: A newborn’s age since birth is zero days. Their gestational age is typically 38 to 42 weeks. Medical professionals use gestational age to assess developmental milestones. If you are calculating age for a baby, confirm which measurement you need. The days between dates calculator can handle the gestational calculation if you know the LMP date and the birth date.

Age in Different Cultures

Not all cultures calculate age the same way. In South Korea, a person is 1 year old at birth, and everyone adds a year on the Lunar New Year, not on their birthday. This system is called Korean age. In 2023, South Korea officially switched to the international system for most legal purposes, but the traditional calculation still appears in social contexts.

In China, age is traditionally counted from conception, though the international system is used for official documents. In Japan, the traditional system (kazoe-doshi) counted age as 1 at birth with a year added on each New Year. It was abandoned in 1902 for the modern system.

What to do: If you are calculating age for international documents, visas, or medical records, always use the method described above. If you are calculating age for cultural ceremonies, check the specific tradition.