Work out half birthday instantly with clear inputs, formula shown and shareable results.
A half birthday falls roughly 182 or 183 days after the birthday - half of 365.25. For a 22 November birthday it lands around 23 May. Half birthdays matter practically for children whose birthdays fall in school holidays, and for age-banded eligibility rules that use half years.
Half birthday
date = birthday + 182.625 days
Days until
days = half birthday - today
Not quite - adding six calendar months gives a different date than adding 182.6 days, because months vary in length. Either convention is used.
To spread celebrations across the term year, so summer-born children are not left out during the holidays.