What this does
Give it a date of birth and a second date — the Age at date field, which starts on today — and it reports the span as full years, then leftover months, then leftover days. Underneath, the same span is repeated as total months, total weeks and total days, plus a countdown to the next birthday. The maths runs in your browser, so the page still works offline and the birth date you type never leaves the tab.
How do I work out my exact age?
- Enter the date of birth in the first field.
- Leave the second field on today, or set it back to get the age as it stood on an earlier date. The picker is capped at today, so future dates are out.
- Read the headline row — years, months, days.
- Use the four boxes below it for Total months, Total weeks, Total days and Next birthday.
There is no submit button; results update as you type. If the second date falls before the birth date you get an error instead of a negative age.
How does it count the months and days?
It advances whole calendar months from the birth date until one more month would overshoot the target date, then counts the days left over. When a month is too short to hold the birth day-of-month, the day is clamped to the last day of that month — 31 January plus one month lands on 29 February in 2024. Whole years are just the month count divided by twelve. Total weeks and total months are floored, never rounded up.
What does a worked example look like?
Born 15 January 1990, measured on 14 January 2020: the answer is
29 years, 11 months and 30 days — not 30 years, because the
birthday is still one day away — with Next birthday: 1 days.
A shorter span shows the clamping: 31 January 2024 to 1 March 2024 reads as
1 month and 1 day. And 1 January to 31 January 2020 gives 30 total days but
only 4 total weeks, since the leftover two days are dropped.
Does it handle a 29 February birthday?
It does, with one quirk worth knowing. Because the day-of-month is clamped, the years figure for a leap-day birthday ticks over on 28 February in ordinary years. The birthday countdown, though, normalises 29 February to 1 March, so on 28 February 2023 someone born on 29 February 2000 sees exactly 23 years and a next birthday one day out. Every other date uses real calendar month lengths, so leap years are counted properly in the day totals.
Why is my age in days higher than years × 365?
Leap days. Thirty calendar years contain seven or eight extra days depending on where the leap years fall, so the total-days figure comes from subtracting the two real dates rather than multiplying. That is also why total months and total days can look out of step with each other — months vary between 28 and 31 days.
What else can I measure?
For a plain gap between two arbitrary dates, use the date difference calculator. To count only business days between them, use work days. If the two dates sit in different countries, time zone converter lines them up first.