A half-birthday is the point exactly midway between one birthday and the next, and this calculator finds it by adding six calendar months to the date of birth. Someone born on 14 March has a half-birthday on 14 September. The idea is mostly a family one — it gives children with birthdays deep in the school holidays or buried in Christmas week a date of their own during term time — but it also turns up in school admissions and childcare bands, where age is sometimes expressed in half years.
Adding six months is not quite as simple as it sounds when the target month is shorter than the starting one. A birthday on 31 August has no 31 February to move to; the convention used here is to fall back to the last day of the shorter month, which for the end of August gives 28 or 29 February. Some tools add 182 or 183 days instead, which for that same date produces a slightly different answer. Neither is more correct; they define the halfway point differently, and this one keeps to the calendar month so that the day number stays recognisable.
The result also shows the weekday and how far away it is, which is the practical part — a half-birthday in the middle of a February half-term is a rather different proposition from one on a school Tuesday.
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