Half-Birthday Calculator

Six months exactly after your birthday — the date shifts if your half-year crosses a shorter month, so let the calendar do the math.

Date of birthAge at date (default: today)

Pick your birth date — every stat computes instantly.

Tip: your result URL is shareable — send it to anyone and they see the same calculation.

How it works — calendar borrowing, not averages

Naive age calculators divide elapsed milliseconds by 30.44 “average” days and drift by a day or two within a year. Real chronological age subtracts like a clerk: borrow days from the true length of the previous month (28, 29, 30, or 31) and months from 12. Born January 31? On March 1 you are 1 month and 1 day old — not 1.03 months. This method is leap-year correct and matches how schools, clinics, and government forms count age.

Formula (years / months)

years = now.y − birth.y − borrow
months = (now.m − birth.m − borrow + 12) % 12

Formula (days)

if days < 0: days += daysInMonth(prev)
months −= 1

Total elapsed

totalDays = round((now@midnight − birth@midnight) / 86400000)
nextBirthday = next occurrence of birth MM/DD

What you get on this page

Hero age — years, months, days in plain English (also readable as Y;M;D for testing kits).
Life totals — total days, weeks (floor(days/7)), hours (days×24), heartbeats (≈ days×86400÷0.857) and breaths.
Next birthday — exact calendar date and days remaining; Feb 29 birthdays map to Feb 28 in common years.
Half-birthday — six months after your birthday, clamped to the target month's length.
Weekday born and shareable URL (?dob=YYYY-MM-DD&asOf=YYYY-MM-DD) for records and messages.

Leap years, Feb 29, and time zones

A year is a leap year if divisible by 4 and (not by 100 unless also by 400). That makes 2024 and 2028 leap years but not 2100. People born on Feb 29 legally celebrate on Feb 28 or Mar 1 in most jurisdictions — we choose Feb 28 for the countdown so the day count never jumps. All day counts are normalized to local midnight, so daylight-saving transitions cannot add or erase a day.

Comparison: exact vs average-month calculators

Method	How it computes months	Jan-31 → Mar-1	Leap accuracy
AgeCalcPro (borrowing)	Borrow actual daysInMonth(prev)	1 month, 1 day ✓	Exact
Avg-month (30.44d)	elapsedDays ÷ 30.44	~0.95 months ✗	Drifts ±2 days
Year-fraction (365.24d)	elapsedDays ÷ 365.24 × 12	~0.96 months ✗	Approximate

Use cases

Official forms — “Age as of…” with a custom as-of date and shareable proof link.
Testing & pediatrics — chronological age in Y;M;D at evaluation date for SLP/OT norms.
Milestones — find your 10k-day anniversary, weeks-old count, or half-birthday party date.
Genealogy & history — compare two birth dates or age at a historical event via the as-of field.

Privacy notice

All math runs in your browser. No birth date is uploaded, stored, or logged. The shareable URL is built client-side with history.replaceState — close the tab and it is gone unless you copy it.

FAQ — Half-Birthday Calculator

How is a half-birthday computed?

Add 6 months to your birth date, clamping the day to the target month's length (e.g., Aug 31 → Feb 28/29).

Why celebrate a half-birthday?

Schools and families use it when the real birthday falls in a break — same 6-month cadence.

Is my data private?

Yes — nothing leaves your device. Bookmark the shareable URL if you want to revisit the same calculation.

Why do other sites show a different age?

Average-month math (30.44 days) drifts ±2 days; borrowing uses the exact previous month length, so results are calendar-correct.

Can I use the share link for records?

Yes — the URL contains ?dob=YYYY-MM-DD&asOf=YYYY-MM-DD and opens the same calculation on any device.

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