Age Calculator

Age is not simply the difference between two calendar years. A person born on 15 June 1990 does not turn 35 until 15 June 2025, even if both dates fall in years that are 35 years apart. Accurate age calculation requires tracking complete anniversaries of the birth date, taking into account varying month lengths, leap years, and whether the birth day has already passed in the current year. This is the standard method used in legal, medical, and administrative contexts worldwide.

The calculation proceeds in three steps. First, the number of complete years is found by counting how many full anniversaries of the birth date have elapsed before the target date. Second, the number of remaining complete months is computed by advancing the birth date by the calculated years and then counting how many full months fit before the target date. Third, the remaining days are found by advancing from the last whole-month mark to the target date. Additionally, the total days elapsed is calculated simply as the difference between the two dates in milliseconds converted to days, and total weeks is derived from that figure.

Practical uses are wide-ranging. Medical professionals use precise age to determine drug dosages for paediatric patients, where the difference between 11 months and 12 months can be clinically significant. Legal systems use exact age to establish eligibility for voting, driving, retirement, and contracts. Genealogists use it to verify historical records. Parents use it to track infant developmental milestones. Immigration authorities use it to verify age-based visa eligibility. The calculator follows the proleptic Gregorian calendar, consistent with ISO 8601, which is the international standard for date representation.

This calculator uses the proleptic Gregorian calendar for all calculations. Dates before the adoption of the Gregorian calendar (15 October 1582 in most Catholic countries; later in others) may not accurately reflect historical age as recorded at the time, since the Julian calendar was in use. People born on 29 February (leap day) are handled correctly — their birthday is considered to fall on 28 February in non-leap years for the purposes of this calculator, which is the most common legal convention, though some jurisdictions use 1 March instead. The calculator does not account for time-of-day or timezone differences; if a person was born late at night and the target date is early morning of what would be their birthday, the result may differ by one day depending on local timezone. For ages spanning very large historical ranges, minor discrepancies may arise from leap year irregularities. This tool is intended for general reference; for legal or medical determinations, always consult the relevant authority or official record.