Half birthday calculator

Type your birthday. The year doesn't change the answer, but it's needed to check the date is real, so `12 January 1982` and `1982-01-12` both work. You get your next half birthday with its day of the week and how many days away it is, the date it falls on every year, and the exact halfway point between your birthdays.

Numeric dates like `03/04/1990` are read with the Date order setting, month first or day first.

The usual half birthday is six months on: born 12 January, half birthday 12 July. It falls on the same date every year, which makes it easy to remember and to put on a calendar.

Six months isn't exactly half a year, though. January to July is 181 days in a common year and the other half is 184. So the calculator also splits the actual days between two birthdays. With 365 days between them, the midpoint is 182.5 days on, at midday; for a 12 January birthday that's 13 July.

Most people use the first. The second is for anyone who wants to be exact.

Six months after 31 August would be 31 February, which doesn't exist. The calculator moves it to the last day of February: the 28th, or the 29th in a leap year. The same happens for birthdays on 29 and 30 August in common years, and on the 31st of March, May, October and December, whose half birthdays land in 30-day months. The working says when it's done this.

Lewis Carroll got there first, more or less. In Through the Looking-Glass (1871), Humpty Dumpty boasts of a cravat he was given as an "un-birthday present", and works out with Alice that there are 364 days in a year for those and only one for birthday presents. A half birthday is a more modest claim on one of them.