How to calculate age and the difference between dates

The number of days between two dates is a simple count. From 1 January to 15 March 2025 is 73 days: 31 for January, 28 for February and 14 more in March.

Months and years are harder because months have different lengths. Express the gap as complete years, then complete months, then leftover days.

Count how many full years have passed since the birth date, then the full months since the last birthday, then the days since that month began. Someone born on 15 January 2000 is 25 years, 2 months and 5 days old on 20 March 2025.

Birthdays on the 29th, 30th and 31st need care. A person born on 29 February has their birthday on 28 February or 1 March in other years, depending on local convention.

Most calculators count the days between dates, so Monday to Tuesday is one day. If you want to include both the first and the last day, as in a hotel booking count of nights versus days, add one.

Check which convention a contract or deadline uses, because it can change the answer by a day.

If you compare exact times rather than dates, daylight saving can make a day 23 or 25 hours long. Using calendar dates avoids this problem for age and deadline calculations.

Our date tools work with calendar dates, so the results do not shift with your time zone.