How old are you, exactly: age, birthdays and eligibility dates

Age is a count of completed years, not a division

Your age in years is the number of full years that have elapsed since your date of birth. You stay the same age from one birthday until the day before the next, which is why the calculation has to know whether this year's birthday has already happened.

It is not days alive divided by 365, and it is not the difference between two years. Both shortcuts give a different answer, and the second one is wrong twice a year for everyone.

Look at the gap between the third and fifth columns. Days divided by 365.25 always returns your age plus the fraction of a year since your last birthday, because that is genuinely what it measures. It is a useful figure for statistics and for tracking infant growth, and it is the wrong figure for anything with a threshold attached.

The fraction never reaches 1, so dividing by 365.25 will not hand you an extra year early. What it does do is overstate your age by up to almost a full year, which is misleading in any context where someone reads the number as an age.

Leap day birthdays, and the one-day question

Someone born on 29 February has a birthday that does not exist in three years out of four. Their age still has to be determined on those years, and there are two conventions in use: treat the anniversary as 28 February, or as 1 March.

The difference is a single day, and it lands exactly on the boundary where it matters most. A person born on 29 February 2008 turns 18 during 2026, a non-leap year. On 28 February 2026 one convention says they are 18 and the other says they are still 17.

• The Age Calculator resolves this by rolling the anniversary forward to 1 March. That is a documented convention, not an accident, and it is stated so the answer can be checked against whatever rule you are actually bound by.
• Where a legal or contractual threshold is involved, the convention that applies is whichever the relevant rule specifies — not whichever one a calculator picked.

If a birthday falls on 29 February and a deadline or entitlement turns on it, find out which convention the rule uses before relying on any calculator, including this one.

Which date does an age limit apply to

This is the trap that catches organisations rather than individuals. A rule saying '18 or over' is incomplete until it says 18 or over on what date.

• The date the application is submitted.
• The date the event or activity takes place.
• The start of the academic or calendar year in which it takes place.
• The date a decision is made.

Those can be months apart, and the same person can be eligible under one reading and ineligible under another. Competitions, insurance, licences and age-banded products all differ in which date they use, and the difference is not clerical — it decides whether a specific person is included.

When a threshold matters, convert it to a date of birth cut-off. 'Must be 18 on 1 September 2026' becomes 'must be born on or before 1 September 2008', which is unambiguous, checkable in a second, and impossible to misread.

Age in months and weeks is a different measure

For the first couple of years, age in years is useless because the milestones are measured in weeks and months. Clinical practice uses completed months, and for newborns, completed weeks — and it uses them precisely because a year-based figure loses the resolution that matters.

This is not a rounding detail. Development assessment windows are expressed in months because a two-month difference at nine months old is significant, while the same difference at twenty-nine is not.

The timezone point, which only bites on the boundary

For 364 days a year, no timezone question arises. On the day of a birthday, or on the day of an eligibility cut-off, the question of whose 'today' it is becomes the whole answer.

A system that computes age using a server clock in one timezone and a user in another can disagree with the user by a day, and it will do so exactly on the boundary dates where the answer is being checked. Which is why the figure should be computed from a stated date rather than from 'now'.

Get the exact figures, and the part that is easy to miss

The Age Calculator returns your age in completed years, months, weeks and days from a date of birth, plus the total days lived and the date of your next birthday — computed on whole calendar dates so the timezone question does not change the answer.