Age Calculator

An age calculator turns a birth date into a clear age result: completed years, remaining months, remaining days, total days lived, and the time until the next birthday. People rely on it for forms, school or activity eligibility, family records, birthday planning, and quick checks where counting by hand is easy to get wrong. Age is more than a simple year subtraction, because the current month and day decide whether the birthday has already passed. Leap years, February 29 birthdays, and month lengths can also affect the result. This tool uses the current browser date as the reference point, so official age limits should still follow the rule stated by the form, school, employer, or authority.

The completed-year age uses a calendar-aware subtraction rather than a raw division by 365.25 days. The algorithm takes today.year - birth.year, then subtracts 1 if today's (month, day) is earlier than the birth (month, day). This matches the Gregorian convention used in civil records: a person born on 1990-06-15 turns 36 exactly on 2026-06-15 at 00:00 local time, not after 36 × 365.25 days. Remaining months and days are computed by borrowing from the next-higher unit using the actual length of the previous month (28 to 31 days), which is why a simple modulo of total days produces off-by-one errors near month boundaries. Total days, weeks, and hours are derived from the millisecond delta (today.getTime() - birth.getTime()) divided by 86,400,000 ms/day. The calculator reads the device's local time, so it implicitly inherits the host's IANA time zone and any active DST offset; this is acceptable for day-level age but can shift the hour count by ±1 across a DST transition. JavaScript's Date object exposes a 0-indexed month (January = 0), which is the most common source of bugs in hand-rolled implementations, and Date.UTC() is preferred when the result must be DST-invariant. The Chinese zodiac is derived from (year - 4) mod 12 against the canonical order Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog, Pig (year 4 CE = Rat). Strictly the cycle rolls over on the lunar new year, not on January 1, so births in January or early February of a Gregorian year may belong to the previous animal under the traditional calendar. The leap-day (February 29) edge case has no global rule: English common law and the UK Births and Deaths Registration Act treat the birthday as March 1 in non-leap years, while many US and Australian jurisdictions use February 28, so the displayed countdown is a default that should be cross-checked against the relevant statute.

• Year-difference algorithm: years = today.year - birth.year - (today.MMDD < birth.MMDD ? 1 : 0), avoiding the 365.25-day approximation that drifts after a leap year
• Millisecond math: total days = floor((Date.now() - birthMs) / 86_400_000); precision is bounded by IEEE 754 doubles to roughly ±0.5 ms across the full Date range of ±100,000,000 days from the epoch
• Gregorian leap-year rule: divisible by 4, except centuries not divisible by 400 (1900 was common, 2000 was leap); the mean year is 365.2425 days, not 365.25
• February 29 birthday: legal birthday in non-leap years defers to jurisdiction (March 1 under UK common law and the Births and Deaths Registration Act 1953 §4; February 28 in many US states; March 1 in New Zealand and several Commonwealth countries); the tool picks a default and surfaces both candidate dates
• Chinese zodiac: animal = ZODIAC[(year - 4) mod 12]; lunar new year falls between Jan 21 and Feb 21, so early-year Gregorian births need a lunar-calendar lookup for full accuracy
• Time zone: Date.getFullYear/getMonth/getDate read the host IANA zone, which can shift the displayed age by one day for births near midnight UTC; Date.UTC variants are used when DST invariance is required
• JavaScript pitfalls: month is 0-indexed (0 = January), new Date('YYYY-MM-DD') parses as UTC while new Date('YYYY/MM/DD') parses as local, and getDay returns 0 for Sunday