Working out someone's exact age, or the time between two dates, sounds easy until months have different lengths and leap years appear. Forms, school admissions, insurance and legal documents often need an exact answer. Here is a reliable method you can use by hand, and the traps to avoid.

Method: Years, Then Months, Then Days

To find the difference between a start date and an end date in years, months and days:

  • Count the full years from the start date to the end date.
  • From that anniversary, count the full months.
  • From that monthly anniversary, count the remaining days.

For example, someone born on 15 March 1990, calculated on 26 October 2026:

  • Full years: 15 March 1990 to 15 March 2026 is 36 years.
  • Full months: 15 March 2026 to 15 October 2026 is 7 months.
  • Days: 15 October to 26 October is 11 days.

The age is 36 years, 7 months and 11 days. In total, that is 13,374 days, or about 1,910 weeks.

When the Day Number Is Smaller

Sometimes the end day is smaller than the start day, so you have to borrow from the previous month. Take a start date of 25 January and an end date of 10 March in the same year. Counting months gives 1 month (to 25 February), and from 25 February to 10 March is 13 days in a non-leap year (3 days to the end of February plus 10). Always count real days using the actual length of the month, never assume 30 days.

Counting Days Between Two Dates

To find the total number of days, subtract the dates, using the number of days in each month. From 1 January to 26 October 2026 is 298 days. Whether you add one depends on the rule you need:

  • Exclusive counting: do not count the start date. From 1 to 3 January is 2 days.
  • Inclusive counting: count both ends. From 1 to 3 January is 3 days.

Legal and contract rules vary, so check which one applies. Spreadsheet software normally subtracts dates exclusively.

Leap Years

A leap year has 366 days, with 29 February added. A year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. So 2024 and 2000 are leap years, but 1900 was not and 2100 will not be.

For people born on 29 February, the legal birthday in non-leap years differs by country: some treat it as 28 February and others as 1 March. If it matters for a legal purpose, check the local rule.

Business Days and Working Time

Calendar days and business days are different. A rough rule is that five out of every seven days are business days, so 28 calendar days contain about 20 business days, before subtracting public holidays. Holidays vary by country and region, so a precise count needs a holiday calendar.

Common Mistakes

  • Assuming every month has 30 days.
  • Forgetting leap days when counting across several years.
  • Mixing inclusive and exclusive counting.
  • Ignoring time zones when two dates are in different places. A birth late in the evening in one country is already the next day in another.

Why Exact Age Matters

Many rules depend on age on a specific date: school entry cut-offs, driving licences, pension eligibility, insurance premiums and sports categories. A difference of a single day can change the answer, so use the exact date rules of the organisation concerned.

Related Skills

Date calculations rely on the same arithmetic as other everyday conversions. Our guide to metric and imperial conversions covers units, and our article on percentages covers the maths behind rates and changes.

Your Next Step

Write your start and end dates, count the full years, then months, then days, and check against the calendar. Use our Age calculator to confirm the result and see the total number of days.