Age Calculator

Age is the time elapsed between a starting date (typically birth) and an ending date (typically today). The standard method counts complete years, then complete months remaining, then days. For example, someone born March 15, 2000 on a date of November 20, 2024 is 24 years, 8 months, and 5 days old — not 24.68 years. The calendar method (used by this calculator and by most legal systems) handles leap years and varying month lengths correctly.

This calculator uses calendar arithmetic, which automatically accounts for leap years. February 29 birthdays roll to March 1 in non-leap years for legal age purposes in most US states, though the convention varies (some states use February 28, and the calculator displays the most common rolling convention). Over a 70-year lifespan, leap years add about 17 extra days compared to a 365-day-per-year approximation — small but not zero.

Calendar age (24 years, 8 months, 5 days) is what people use socially and legally. Decimal age (24.68 years) is what scientists and statisticians sometimes use — total days divided by 365.25. The two answer slightly different questions: calendar age is exact for human conventions; decimal age is more useful for comparing populations or running time-based calculations. This calculator shows both.

If you were born at 11pm Eastern time and ask about your age at 5am Eastern the next day, you're a day older. But if someone in Tokyo (14 hours ahead) does the calculation, they might already see the next day on their clock when you're still on your birthday. This calculator uses your local time zone — which is what almost everyone wants. For legal documents or international filings, your birth jurisdiction's time zone is typically the authoritative one.

Enter your birth date — the calculator displays the day of the week automatically. The algorithm uses Zeller's congruence or a similar calendar formula. Curiously, the day of the week your birthday falls on cycles through all 7 days roughly every 28 years (with leap year adjustments), so if you were born on a Tuesday, you'll celebrate on a Tuesday again 11, 6, 5, and 6 years later in repeating patterns.

Practical uses include precise medical dosing (pediatric medications, gestational age post-birth), counting days for legal eligibility (driving, voting, drinking, retirement age), milestone celebrations (10,000 days alive ≈ 27 years, 4 months), and athletic record categories. Some cultures track age in days for infants — a baby '60 days old' rather than '2 months' is more precise.

A half-birthday falls exactly 6 months from your birth date. For most months this is simple (March 15 → September 15). For dates near the end of a month (August 31 → no February 31, so February 28 or 29). Half-birthdays are popular for kids whose birthdays fall in school holidays (so they can celebrate during the school year) and as informal milestones. The calculator can compute your half-birthday by setting the end date 6 months from any birth date.

Yes. The 'from date' and 'to date' don't have to be a birthday and today. Use it to calculate the gap between any two dates: time at a company (hire date to today), duration of a project, length of a relationship, time since an event. The math is identical — calendar arithmetic between two timestamps.

Calendar months have different lengths (28 to 31 days), so 'X years and Y months' doesn't convert cleanly to total months. The calculator's 'total months' figure rounds to complete calendar months, which may be 1–2 days off from years × 12. The most precise figure is total days, which has no ambiguity. Total weeks is total days divided by 7.

Chronological age is what this calculator measures: actual elapsed time since birth. Biological age is an estimate of how 'aged' your body is based on health markers — telomere length, epigenetic clocks, cardiovascular markers, organ function. Biological age can be older or younger than chronological age and is roughly 30–40% genetic and 60–70% lifestyle (sleep, diet, exercise, stress, smoking). Various blood-based biological age tests exist commercially but most aren't yet medical-grade.