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Age Difference Calculator

Calculate the exact difference between two dates as years / months / days / hours / minutes / seconds and as a total in every unit (years, months, weeks, days, hours, minutes, seconds, milliseconds). Live mode counts up in real time. Next anniversary helper with Feb 29 handling. Free and private.

What is the Age Difference Calculator?

An age-difference calculator returns the exact gap between two moments in time. Toollyz Age Difference Calculator runs the math entirely in your browser using calendar-correct subtraction — the same convention Day.js's `.diff` uses, where 30 Jan + 1 month = 28 Feb in a non-leap year. Two `datetime-local` inputs accept the from and to dates down to the minute, and the result is shown two ways: as a six-part breakdown (years, months, days, hours, minutes, seconds) in the hero, and as totals in eight units (decimal years using the Gregorian mean of 365.2425 days/year, months, weeks, days, hours, minutes, seconds and milliseconds). Live mode updates the `To` field once per second so a birthday counter ticks up in real time. The Next Anniversary helper computes the upcoming celebration with Feb-29 handling — in non-leap years the anniversary falls on Feb 28 of that year and snaps back to Feb 29 the following leap year. The current pair persists in localStorage; Toollyz has no backend.

How to use it

  1. Pick your From date (e.g. a birthday) and a To date (defaults to now).
  2. Watch the six-part breakdown and total-in-every-unit panel update.
  3. Leave Live mode on for a real-time counter, or turn it off to compute a fixed gap.
  4. Click Copy summary for a shareable line, or Swap to invert the dates.

Benefits

  • Calendar-correct breakdown: years / months / days / hours / minutes / seconds in the hero.
  • Totals in eight units (years, months, weeks, days, hours, minutes, seconds, milliseconds).
  • Decimal years using the Gregorian mean of 365.2425 days per year.
  • Live mode updates the To field once per second for a real-time counter.
  • Next-anniversary helper with explicit Feb 29 handling.
  • Datetime-local inputs preserve your timezone — no UTC surprises.
  • Reversed-direction detection — the calculator shows the absolute difference and tells you the dates were swapped.
  • 100% private — Toollyz has no backend, dates save to localStorage on this device only.

Frequently asked questions

How does the calendar-correct subtraction work?

Toollyz subtracts month-by-month, borrowing days from the prior month when needed. For example, going from 30 Jan 2026 to 28 Feb 2026 is 0 years, 1 month, 0 days (a full calendar month) — not 29 days. This matches what Day.js's `.diff` and ICU's relative-time formatters do.

Why is decimal years slightly different from `years + months/12`?

Pure division ignores leap days. Decimal years uses the Gregorian mean of 365.2425 days per year (the average length when leap years are counted properly), so a span of 1 year and 1 day reports as 1.003 years rather than 1.000.

What happens with Feb 29 birthdays?

The Next Anniversary helper detects a Feb 29 birthday and bumps it to Feb 28 in non-leap years — the most widely-used convention. The following leap year, it lands on Feb 29 again. Both behaviours match how most calendars handle leap-day birthdays.

Does it handle timezones?

The inputs are `datetime-local`, which means they're interpreted in your browser's local timezone. Travelling between timezones doesn't change the calculation — only the wall-clock values you typed do. For UTC math, paste an ISO string starting with `Z` into the address bar or the Unix Timestamp Converter.

What's the maximum precision?

The breakdown is to the second; the totals include milliseconds. Browsers' `Date` API is millisecond-precision, so you can't go finer than that without `performance.now()`-based tools (try the Stopwatch).

Why does Live mode tick once per second?

A 1 Hz update is enough for a human-readable age counter without churning the CPU. The underlying calculation is millisecond-precise; only the display is throttled.

Can I swap the dates?

Yes — there's a swap button between the two inputs. If From is later than To, the calculator detects it automatically, computes the absolute difference and shows a banner telling you the order is reversed.

Are my dates saved across devices?

No — they're stored in localStorage on this device only. Toollyz has no backend; nothing is uploaded or synced.

What about the time of day?

Both inputs accept hours and minutes. The breakdown and totals account for them — a birthday at 08:30 will show 0 hours when the current time is 08:30, even if the dates haven't changed.

Is this Age Difference Calculator free?

Completely free with no signup and no limits. Calculate as many spans as you like — privately in your browser.

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