Age & Date Calculator
Calculate exact age from a date of birth, and find the difference between any two dates in years, months, and days.
Results are for general guidance only — not professional advice. Learn more.
How to use this tool
Choose a calculation mode, enter your dates, and results update instantly. The tool offers three modes:
- Age Calculator: Enter a date of birth and a target date to find the exact age in years, months, and days, total months and days, and the next birthday.
- Date Difference: Enter two dates to find the total number of days, weeks, and hours between them.
- Add / Subtract Days: Enter a start date and a number of days (positive to add, negative to subtract) to find the resulting date and day of the week.
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Frequently asked questions
How is age calculated exactly?
Age is calculated by counting the number of complete years, months, and days between a birth date and a target date. A new year of age begins on the birthday each year. If the birthday has not yet occurred in the current calendar year, one year is subtracted from the difference in calendar years.
What is a Unix timestamp?
A Unix timestamp is the number of seconds elapsed since 1 January 1970 00:00:00 UTC, known as the Unix epoch. It is a common way to represent points in time in computing systems because it is a simple integer that is timezone-independent. JavaScript uses milliseconds since the Unix epoch internally.
How many days are in a year on average?
The average Gregorian calendar year is 365.2425 days, accounting for leap years (366 days every 4 years, with exceptions for century years). This is why age calculations can show slightly different day counts depending on how leap years fall within the period.
How do I calculate the difference between two dates?
To calculate the number of days between two dates, subtract the earlier date from the later date in milliseconds and divide by 86,400,000 (the number of milliseconds in a day). For years, months, and days, you need to account for the variable lengths of months and leap years.