How to Calculate Exact Age in Years, Months and Days

Why Simple Year Subtraction Fails

The naive formula — current year minus birth year — gives wrong results in two cases: (1) when your birthday hasn't occurred yet this year, and (2) whenever you need months and days, not just years.

Example: Someone born September 15, 1990. On September 1, 2025: they are still 34, not 35 (birthday not yet reached). Simple subtraction (2025 - 1990 = 35) is wrong by 1 year.

The Correct Algorithm (Step by Step)

Birth date: March 15, 1990
Today's date: February 10, 2025

Step 1: Years
  Current year - birth year = 2025 - 1990 = 35
  But Feb 10 is before Mar 15, so birthday hasn't happened yet
  Years = 35 - 1 = 34

Step 2: Months
  Current month - birth month = 2 - 3 = -1
  Since negative, add 12 and we already subtracted a year above
  Months = -1 + 12 = 11

Step 3: Days
  Days in previous month (January) = 31
  Current day + (days in last month - birth day)
  = 10 + (31 - 15) = 10 + 16 = 26

Result: 34 years, 11 months, 26 days

Handling Leap Years

• A year is a leap year if: divisible by 4, except if divisible by 100, except if divisible by 400
• 2000 = leap year (divisible by 400)
• 1900 = NOT leap year (divisible by 100 but not 400)
• 2024 = leap year (divisible by 4, not 100)
• February 29 birthdays: treated as March 1 in non-leap years for most legal purposes

JavaScript Implementation

function calculateAge(dob) {
  const birth = new Date(dob);
  const now = new Date();
  
  let years = now.getFullYear() - birth.getFullYear();
  let months = now.getMonth() - birth.getMonth();
  let days = now.getDate() - birth.getDate();
  
  if (days < 0) {
    months--;
    days += new Date(now.getFullYear(), now.getMonth(), 0).getDate();
  }
  if (months < 0) {
    years--;
    months += 12;
  }
  
  return { years, months, days };
}