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

Working out your age in whole years is easy. Getting your exact age in years, months, and days, or calculating the precise gap between two dates, is deceptively tricky because months have different lengths and leap years add an extra day.

Why Age Calculation Is Harder Than It Looks

A naive approach subtracts one year from another, but that ignores whether the birthday has already occurred this year. Months compound the problem: going from January 31 to February involves a 28, 29, 30, or 31-day month depending on the year. Any accurate method has to account for all of this.

The Manual Method

To calculate an exact age from a birth date to today, subtract each component and borrow when a value goes negative:

• Subtract the birth day from today's day. If negative, borrow days from the previous month.
• Subtract the birth month from the current month. If negative, borrow 12 months and reduce the year by one.
• Subtract the birth year from the current year.
• The result is your age in years, months, and days.

Leap Years and Edge Cases

A leap year occurs every 4 years, except century years not divisible by 400 (so 1900 was not a leap year, but 2000 was). People born on February 29 have a birthday only once every four years, which is why age tools must handle this date carefully rather than rounding it to March 1 or February 28.

// Difference in whole days between two dates
const birth = new Date('1998-06-20');
const today = new Date();
const days = Math.floor((today - birth) / (1000 * 60 * 60 * 24));
console.log(days + ' days old');

For anything official, such as eligibility or legal age, always compute the exact date difference rather than subtracting years, since a birthday days away can change the result.

The Age Calculator handles leap years, month lengths, and edge cases automatically, giving you an exact age or date gap in seconds.