Age Calculator
Your age, down to the day
Years, months and days between two dates, plus total weeks, days and hours, and the next birthday countdown.
Age Calculator
Runs in your browser · nothing uploadedProgress through the current year of life. Bar chart with 12 steps across the term, climbs from 8.333333333333332% at the first step, to 58.333333333333336% at the midpoint, to 100% by the final step.
01 — How to use Age Calculator
02 — Getting good results
03 — Questions
No. The whole calculation is arithmetic done in this browser tab. A date of birth is personal data, and this tool never transmits or stores it.
Because there is no single correct rule for spans that cross a month with a different length. Compare 31 January to 28 February: this tool reports one month, on the basis that one month after 31 January is the last day of February. Other tools report 28 days. Neither is wrong, but they disagree.
Exactly, using the full Gregorian rule — divisible by four, except centuries, unless divisible by four hundred. That is why 1900 was not a leap year and 2000 was.
Because a date of birth as normally recorded has no time attached, and an age reported to the second from a date-only input would be fabricated precision. The tool works in whole days and says so.
04 — More about Age Calculator
Why calendar arithmetic is harder than it looks
Measuring elapsed time in days is trivial: subtract two timestamps and divide. Measuring it in years, months and days is not, because months have no fixed length and the answer depends on rules nobody wrote down. What is one month after 31 January? There is no 31 February, so the answer must be chosen: the last day of February, or the first of March, or an error. What is the age of someone born on 29 February, on 28 February in a non-leap year? Different jurisdictions answer differently for legal purposes. The practical consequence is that any implementation has to pick a convention, and implementations that pick silently produce answers that disagree with each other in exactly the cases people notice. This tool clamps a day past the month end back to the last day of that month, and treats 29 February as 1 March in non-leap years — stated here because there is no way to make the choice invisible.
The borrowing bug
The usual way to compute a calendar difference is to subtract each field and borrow when one goes negative — much like long subtraction. It is also where most implementations break, because the number of days available to borrow depends on which month you borrow from, and it is easy to borrow from the wrong one. Worse, some spans cannot be fixed by a single borrow at all: from 31 January to 1 March, the day difference is minus thirty, and February offers only twenty-eight days to borrow, leaving the result still negative. Implementations that assume one borrow always suffices return a negative day count for those dates. Stepping the start date forward one month at a time until it passes the end, then counting the remaining days, avoids the problem entirely because it never produces a negative to fix.
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Same shell, same shortcuts, same promise: nothing leaves your browser.