Age Calculator Guide

An age in years changes on a birthday, not every time another 365 days pass. Age Calculator compares a birth date with the date you choose under Age as of.

Try a birthday

Set Date of birth to 2000-01-01 and Age as of to 2024-01-01. Under Result, Age should say 24 years, 0 months, 0 days. The birthday happened on the date you chose. Next birthday should say today for this example because it is measured as of that date, even if the real date today is different.

Now change Age as of to 2023-12-31. The person has not reached the 2024 birthday yet, so the age should be 23 years, plus months and days. The tool also shows total days, weeks, and hours; those are ways to measure the same span, not separate ages.

If it refuses your dates

The birth date must be on or before Age as of. The second date starts at today, but you can choose a past date for a record or a future date for planning. For an official form, check which date it asks you to calculate the age on. Do not put a real person’s date of birth in a public screenshot unless they agreed to share it.

Where your input goes

Processing runs in your browser. The tool does not upload your input to ilham.dev.

For more background, read Dates, Timestamps, and Time Zones.

Questions you might have

Why not just divide the total days by 365?: Because a year is not 365 days on average and months have different lengths. Counting whole years, then months, then days is how people read an age, and it stays correct across leap years.
Which date is the second field for?: It is the date you are measuring the age at. It defaults to today, but you can set it to a past date for a record or a future date to see how old someone will be.
How is the next birthday counted?: The page takes the birth month and day in the current year, or the next one if that has already passed this year, and counts the days between. A birthday today shows as today.