Age in Weeks Calculator

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.

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

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.