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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →An age calculator is a calendar comparison, not a division of elapsed milliseconds. To get a correct count of completed years, keep the birth date and the as-of date as year, month and day values, subtract the years, and subtract one more if the birthday has not yet arrived in the as-of year. Leap years only affect a few edge cases, but the most important one, a February 29 birthday, needs an explicit policy in your code.
Build order at a glance
- Read the date input as a plain
yyyy-mm-ddstring and split it into integers. - Reject an empty, malformed or impossible date, such as 1900-02-29.
- Capture the as-of date once, at the start of the calculation.
- Reject a birth date that falls after the as-of date.
- Compute completed years, then the optional months and days remainder.
- Write the result to a status region and keep the entered date visible.
Start with calendar dates, not timestamps
A person’s birthday is a calendar date with no time of day and no time zone attached. The HTML date input works the same way. According to MDN’s documentation for <input type=”date”>, the control provides a date without a time, and its value is always normalized to yyyy-mm-dd. The picker the user sees varies by browser and operating system, but the string your script receives does not. Parse the string you receive, not the text the user sees.
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Why passing the string to new Date can lose a day
A JavaScript Date is a millisecond time value counted from 1970-01-01 UTC, so it represents a moment rather than a birthday. As described in MDN’s Date reference, a date-only ISO string is interpreted as UTC midnight. If you then read it back with local getters such as getDate(), a user in a time zone west of UTC sees the previous calendar day. The error happens before any age arithmetic begins, so the age can be wrong even when the arithmetic is correct.
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| Approach | What it represents | Strength | Risk |
|---|---|---|---|
| Integer year, month and day | A civil calendar date | The birthday comparison is explicit, with no time zone involved | You write the validation and leap-year checks yourself |
JavaScript Date built from a date string |
An instant, in milliseconds since the epoch | Built-in formatting and date methods | Local and UTC reads can change the day shown |
Temporal.PlainDate |
A calendar date with no time or zone | Designed for date-only values | Browser support must be checked before use; see MDN’s Temporal page |
Leap years: the rule and where it matters
MDN’s guide to date and time formats in HTML states the Gregorian rule plainly: “A leap year is any year which is divisible by 400 or the year is divisible by 4 but not by 100.” Under that rule 2024 is a leap year, 1900 is not (it is divisible by 100 but not by 400), and 2000 is (it is divisible by 400). February has 29 days in a leap year and 28 otherwise.
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const MONTH_DAYS = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
function isLeapYear(year) {
return (year % 400 === 0) || (year % 4 === 0 && year % 100 !== 0);
}
function daysInMonth(year, month) {
return month === 2 && isLeapYear(year) ? 29 : MONTH_DAYS[month - 1];
}Leap years affect age calculation in two places: validation (is 1900-02-29 a real date?) and the anniversary of a February 29 birthday. The first is a simple check. The second requires a decision.
February 29 birthdays and a leap-day policy
MDN’s documentation does not define how a February 29 birthday should be handled in a non-leap year, so your calculator has to choose. Two policies are workable. Under the February 28 policy, the anniversary in a common year falls on February 28. Under the March 1 policy, it falls on March 1. Neither is a universal standard, so state the one you use in the interface or documentation.
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Take a person born on 2000-02-29. The completed age depends on the policy and the as-of date:
| As-of date | February 28 policy | March 1 policy |
|---|---|---|
| 2023-02-27 | 22 | 22 |
| 2023-02-28 | 23 | 22 |
| 2023-03-01 | 23 | 23 |
| 2024-02-29 | 24 | 24 |
These values follow from the calendar arithmetic described below. They are derived results, not figures quoted from a standard.
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Counting completed years
The age in completed years is the year difference, minus one if the anniversary in the as-of year has not yet been reached. The comparison uses month and day, and it treats the anniversary date as reached when the two dates match, which is how a birthday today counts as a completed year.
function compareDates(a, b) {
return (a.year - b.year) || (a.month - b.month) || (a.day - b.day);
}
// Returns the anniversary of the birth date in a given year, applying the leap-day policy.
function anniversaryIn(year, birth, policy) {
if (birth.month === 2 && birth.day === 29 && !isLeapYear(year)) {
return policy === 'mar1'
? { year, month: 3, day: 1 }
: { year, month: 2, day: 28 };
}
return { year, month: birth.month, day: birth.day };
}
function completedYears(birth, asOf, policy = 'feb28') {
let years = asOf.year - birth.year;
const anniversary = anniversaryIn(asOf.year, birth, policy);
if (compareDates(asOf, anniversary) < 0) years -= 1;
return years;
}Notice what the function does not do. It never converts days to years by dividing by 365.25. That approach drifts around leap years and leaves a birthday’s arrival dependent on rounding, while the calendar comparison gives an exact answer.
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Months and days remainder
If you also display months and days, measure them from the most recent anniversary rather than from the birth date. Subtract the calendar fields, and when the day count goes negative, borrow from the month before the as-of month using that month’s actual length.
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function yearsMonthsDays(birth, asOf, policy = 'feb28') {
const years = completedYears(birth, asOf, policy);
const anchor = anniversaryIn(birth.year + years, birth, policy);
let months = (asOf.year - anchor.year) * 12 + (asOf.month - anchor.month);
let days = asOf.day - anchor.day;
if (days < 0) {
months -= 1;
const prevMonth = asOf.month === 1 ? 12 : asOf.month - 1;
const prevYear = asOf.month === 1 ? asOf.year - 1 : asOf.year;
days += daysInMonth(prevYear, prevMonth);
}
return { years, months, days };
}For a birth date of 1990-01-31 and an as-of date of 2024-03-15, the anchor is 2024-01-31. The result is 34 years, 1 month and 13 days. The 13 comes from borrowing February’s 29 days. Label this output as a calendar interval, not a decimal age. Month-end cases are a convention, and other calculators may resolve them differently, so document the rule you choose.
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Validation and the modern UI
Markup
<label for='birth-date'>Date of birth</label>
<input id='birth-date' name='birth-date' type='date' required
aria-describedby='birth-error'>
<p id='birth-error' hidden></p>
<p id='result' role='status' aria-live='polite'></p>
<button type='button' id='reset'>Start over</button>A visible label is required because the picker’s appearance is controlled by the browser and operating system, so the label is the only consistent cue for the field’s purpose.
Validation rules
- Require a value. An empty field produces a message, not a result.
- Check that the string is a real Gregorian date with
daysInMonth, so 2023-02-29 and 1900-02-29 are rejected. - Reject any birth date after the as-of date. For a conventional current-age calculator, this covers future dates.
- Set
minandmaxon the input where you want the picker to restrict choices. These attributes only limit the control, so the same checks must also run in your code. - Show errors as text next to the field and link them with
aria-describedby. Do not rely on red color alone. - When validation fails, keep the entered value in the field so the user can correct it.
Announcing the result
Write the result into the element with role='status' and aria-live='polite' once per calculation, not on every keystroke. A polite live region lets assistive technology read the new age without interrupting what the user is doing. The reset button should clear the result, hide the error and return focus to the date field.
Testing the calculator
These cases exercise the boundaries that most often break a hand-written calculator. The expected values assume the as-of date is 2026-10-08 where a row says so.
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| Birth date | As-of date | Expected completed years | What it checks |
|---|---|---|---|
| 1990-10-08 | 2026-10-08 | 36 | A birthday today counts as reached |
| 1990-10-09 | 2026-10-08 | 35 | A birthday tomorrow has not yet occurred |
| 1990-10-07 | 2026-10-08 | 36 | A birthday yesterday has passed |
| 2000-02-29 | 2024-02-29 | 24 | A leap day exists in the as-of year, so both policies agree |
| 2000-02-29 | 2023-02-28 | 23 (February 28 policy) or 22 (March 1 policy) | The leap-day policy decides the result |
| 2000-02-29 | 2026-10-08 | 26 | Both policies agree once the anniversary has passed |
| 1900-02-29 | any | Rejected as invalid | 1900 is divisible by 100 but not by 400 |
Why is my age calculator off by one?
When an age is off by one, the cause is usually one of the following:
- The date string was passed to
new Dateand read back with local getters, which shifts the day in time zones west of UTC. - The as-of date was read from the clock more than once, so the calculation straddles midnight.
- The code subtracts the years but skips the check for whether the anniversary has been reached.
- The age was derived by dividing elapsed days by 365.25 and then rounding or truncating, which can land on the wrong side of a birthday.
- A February 29 birthday uses an unstated leap-day policy, so the result changes between March 1 and February 28 without warning.
Where Temporal fits
Temporal’s calendar date types represent a date separately from any time zone, which matches the birthday use case closely. Before you rely on them in production, confirm current browser support on MDN’s Temporal reference. For a small calculator, plain integer year, month and day values are enough and need no polyfill.
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