How old are you,
to the day?

Field guide

How age is actually calculated.

On the surface, age is simple subtraction: today's year minus the year you were born. In practice, almost every culture also wants the months and days. Doing that correctly takes a little care — months don't all have the same number of days, and the calendar is full of small irregularities that trip up naive arithmetic.

The standard “years, months, days” method

The everyday English-speaking convention is to count completed years, then completed months, then the leftover days. The algorithm has three steps:

1. Subtract the birth year from the as-of year — that's your initial year count.
2. Subtract birth month from as-of month. If the result is negative, “borrow” 12 months and subtract one year.
3. Subtract birth day from as-of day. If the result is negative, borrow the number of days in the previous month and subtract one month.

Worked example

Born 1995-06-15, today is 2024-03-08.

• Years: 2024 − 1995 = 29
• Months: 3 − 6 = −3 → borrow 12, year drops to 28. Months = 9
• Days: 8 − 15 = −7 → borrow February (29 days in 2024) → days = 22, months drops to 8
• Final: 28 years, 8 months, 22 days.

Why this is harder than it looks

Months are not a fixed unit. February has 28 days (or 29 in a leap year), and the others swing between 30 and 31. When you borrow, you have to use the number of days in the month you're borrowing from, not a generic 30. That's the bug in most homemade age calculators.

Leap years

A year is a leap year if it's divisible by 4, except century years, which must also be divisible by 400. So 2000 was a leap year, 1900 wasn't, and 2100 won't be. People born on February 29 (about 1 in 1,461) celebrate their “official” birthday on either February 28 or March 1 in non-leap years, depending on jurisdiction.

Different cultures, different counts

Not every culture counts age the same way. The traditional East Asian system counts you as 1 year old at birth and adds a year on the lunar new year, so a baby born December 31 might be “2 years old” on January 2 by Korean reckoning. Korea formally adopted the international system for legal purposes in June 2023, but the older count persists culturally.

Total counts: days, weeks, hours, minutes

Once you have the millisecond gap between two dates, the other units are arithmetic:

We use floor for days because partial days don't round up — you haven't lived through 11,000 days until the full 11,000th day has elapsed.

Total months: an approximation

Total months is reported as years × 12 + months from the calendar diff. It rolls leftover days off the end, which is the same convention banks and credit reports use (“account opened 47 months ago”).

Day of the week

Knowing which day of the week you were born is a small party trick that the calculator throws in. It's computed directly from the JavaScript Date object, which uses the proleptic Gregorian calendar — fine for any modern date, but produces “wrong” answers for dates before 1582 in countries that hadn't yet adopted Gregorian.

Practical uses

• Birthday planning: see exactly how many days until your next milestone.
• Forms & eligibility: verify you're over 18, 21, or 65 on a specific date.
• Anniversaries: change “date of birth” to the day you started a job, met someone, or moved house, and the as-of date stays today.
• Pets: works the same for any creature with a birthdate, dog years optional.

A note on time zones

This calculator treats both dates as plain calendar dates, without time-of-day or timezone semantics. That's the right behavior for “how old am I” — what matters is the date in your local calendar, not the precise moment of birth.