Half-Birthday Calculator
A half-birthday is the day exactly six months after your birthday — born August 15, your half-birthday lands on February 15. Enter a date of birth and the tool shows the next half-birthday's date and weekday, the days until it, and the half-age you will reach (e.g. 33.5 years).
Please enter a valid number
Today is your half-birthday — happy half-birthday!
Next half-birthday
—
Half-age you will turn
— years
Previous half-birthday
—
Half-age you turned
— years
The target month has fewer days (e.g. Aug 31 → late Feb) — the half-birthday is shifted to that month's last valid date.
Formula
half-birthday = date_of_birth + 6 calendar months (wrapping into the next year if needed; if the target month has fewer days the date snaps to the last valid day). The n-th half-birthday is birth + (2n + 1) × 6 months, with half-age = n + 0.5.
- · Months are added with calendar arithmetic: Aug 15 + 6 months = Feb 15; Jul 1 + 6 months = Jan 1 of the next year.
- · Day clamping: an Aug 31 birthday would land on the non-existent Feb 31 — the calculator snaps to Feb 28 (non-leap) or Feb 29 (leap). Likewise Mar 31 → Sep 30.
- · Feb 29 babies: the half-birthday is always Aug 29 — August has 31 days, so no clamping is needed. (The day-of-birth itself is what causes the usual Feb 28 vs Mar 1 debate, not the half-birthday.)
- · Half-age: 0 at birth, 0.5 at the first half-birthday, 1 at the next birthday, 1.5 at the next half-birthday, and so on. The n-th half-birthday lands at half-age n + 0.5.
- · Day counts use UTC to avoid timezone and daylight-saving drift.
Frequently asked
What is a half-birthday?
Your half-birthday is the date six months after your actual birthday — the day you reach a half-integer age like 0.5, 1.5, 2.5 years. In the US it is a common workaround for kids whose real birthdays fall in summer holidays, and adults use it as a "halfway-point" milestone.
When is the half-birthday for someone born on Aug 31?
It lands at the end of February. Aug 31 + 6 months = Feb 31, which does not exist, so the calculator snaps to Feb 28 (non-leap year) or Feb 29 (leap year). The widget shows a 'clamped' note when this happens.
How does the half-birthday work for leap-day births (Feb 29)?
Feb 29 + 6 months = Aug 29 — a date that exists every year, so no clamping is needed. Leap-day babies actually get a more consistent half-birthday than their actual birthday.
What time zone does the calculator use? Do hours matter?
The calculator works at the day granularity (YYYY-MM-DD) and uses UTC for day arithmetic, so daylight-saving changes and time zones do not shift the result — you see the same answer whether you load the page from Hong Kong, New York or London.
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