There’s not much magic in calculating your half-birthday, although there are two different methods. The first, which is the simplest, is to take your birthday and add six months; for example, someone born on January 1 would have a half-birthday of July 1. That means that some unlucky people have no half-birthday!
The second way—this site’s method—is to add half a year, i.e. 182.5 days.